diff --git a/packages/core/ui-settings/core-ui-settings-common/index.ts b/packages/core/ui-settings/core-ui-settings-common/index.ts
index c06251643fbf7..d290b9065c546 100644
--- a/packages/core/ui-settings/core-ui-settings-common/index.ts
+++ b/packages/core/ui-settings/core-ui-settings-common/index.ts
@@ -17,6 +17,18 @@ export type {
   GetUiSettingsContext,
 } from './src/ui_settings';
 export { type DarkModeValue, parseDarkModeValue } from './src/dark_mode';
-export { parseThemeNameValue } from './src/theme_name';
+export {
+  DEFAULT_THEME_TAGS,
+  SUPPORTED_THEME_TAGS,
+  DEFAULT_THEME_NAME,
+  SUPPORTED_THEME_NAMES,
+  FALLBACK_THEME_TAG,
+  parseThemeTags,
+  hasNonDefaultThemeTags,
+  parseThemeNameValue,
+  type ThemeName,
+  type ThemeTag,
+  type ThemeTags,
+} from './src/theme';
 
 export { TIMEZONE_OPTIONS } from './src/timezones';
diff --git a/packages/core/ui-settings/core-ui-settings-common/src/theme.ts b/packages/core/ui-settings/core-ui-settings-common/src/theme.ts
new file mode 100644
index 0000000000000..ca89958deac16
--- /dev/null
+++ b/packages/core/ui-settings/core-ui-settings-common/src/theme.ts
@@ -0,0 +1,97 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+export const DEFAULT_THEME_NAME = 'amsterdam';
+export const SUPPORTED_THEME_NAMES = ['amsterdam', 'borealis'];
+
+export type ThemeName = typeof SUPPORTED_THEME_NAMES[number];
+
+/**
+ * Theme tags of the Amsterdam theme
+ */
+export const ThemeAmsterdamTags = ['v8light', 'v8dark'] as const;
+
+/**
+ * Theme tags of the experimental Borealis theme
+ */
+export const ThemeBorealisTags = ['borealislight', 'borealisdark'] as const;
+
+/**
+ * An array of all theme tags supported by Kibana. Note that this list doesn't
+ * reflect what theme tags are available in a Kibana build.
+ */
+export const SUPPORTED_THEME_TAGS = [...ThemeAmsterdamTags, ...ThemeBorealisTags] as const;
+
+export type ThemeTag = (typeof SUPPORTED_THEME_TAGS)[number];
+export type ThemeTags = readonly ThemeTag[];
+
+/**
+ * An array of theme tags available in Kibana by default when not customized
+ * using KBN_OPTIMIZER_THEMES environment variable.
+ */
+export const DEFAULT_THEME_TAGS: ThemeTags = ThemeAmsterdamTags;
+
+export const FALLBACK_THEME_TAG: ThemeTag = 'v8light';
+
+const isValidTag = (tag: unknown) =>
+  SUPPORTED_THEME_TAGS.includes(tag as (typeof SUPPORTED_THEME_TAGS)[number]);
+
+export function parseThemeTags(input?: unknown): ThemeTags {
+  if (!input) {
+    return DEFAULT_THEME_TAGS;
+  }
+
+  if (input === '*') {
+    // TODO: Replace with SUPPORTED_THEME_TAGS when Borealis is in public beta
+    return DEFAULT_THEME_TAGS;
+  }
+
+  let rawTags: string[];
+  if (typeof input === 'string') {
+    rawTags = input.split(',').map((tag) => tag.trim());
+  } else if (Array.isArray(input)) {
+    rawTags = input;
+  } else {
+    throw new Error('Invalid theme tags, must be an array of strings');
+  }
+
+  if (!rawTags.length) {
+    throw new Error(
+      `Invalid theme tags, you must specify at least one of [${SUPPORTED_THEME_TAGS.join(', ')}]`
+    );
+  }
+
+  const invalidTags = rawTags.filter((t) => !isValidTag(t));
+  if (invalidTags.length) {
+    throw new Error(
+      `Invalid theme tags [${invalidTags.join(', ')}], options: [${SUPPORTED_THEME_TAGS.join(
+        ', '
+      )}]`
+    );
+  }
+
+  return rawTags as ThemeTags;
+}
+
+export const hasNonDefaultThemeTags = (tags: ThemeTags) =>
+  tags.length !== DEFAULT_THEME_TAGS.length ||
+  tags.some((tag) => !DEFAULT_THEME_TAGS.includes(tag as (typeof DEFAULT_THEME_TAGS)[number]));
+
+export const parseThemeNameValue = (value: unknown): ThemeName => {
+  if (typeof value !== 'string') {
+    return DEFAULT_THEME_NAME;
+  }
+
+  const themeName = value.toLowerCase();
+  if (SUPPORTED_THEME_NAMES.includes(themeName.toLowerCase() as ThemeName)) {
+    return themeName as ThemeName;
+  }
+
+  return DEFAULT_THEME_NAME;
+};
diff --git a/packages/core/ui-settings/core-ui-settings-common/src/theme_name.ts b/packages/core/ui-settings/core-ui-settings-common/src/theme_name.ts
deleted file mode 100644
index 2e6043d098d66..0000000000000
--- a/packages/core/ui-settings/core-ui-settings-common/src/theme_name.ts
+++ /dev/null
@@ -1,16 +0,0 @@
-/*
- * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
- * or more contributor license agreements. Licensed under the "Elastic License
- * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
- * Public License v 1"; you may not use this file except in compliance with, at
- * your election, the "Elastic License 2.0", the "GNU Affero General Public
- * License v3.0 only", or the "Server Side Public License, v 1".
- */
-
-export const parseThemeNameValue = (value: unknown): string => {
-  if (typeof value !== 'string') {
-    return 'amsterdam';
-  }
-
-  return value;
-};
diff --git a/packages/core/ui-settings/core-ui-settings-server-internal/src/settings/theme.ts b/packages/core/ui-settings/core-ui-settings-server-internal/src/settings/theme.ts
index b2c7e6029cf93..d45d89f4e4af5 100644
--- a/packages/core/ui-settings/core-ui-settings-server-internal/src/settings/theme.ts
+++ b/packages/core/ui-settings/core-ui-settings-server-internal/src/settings/theme.ts
@@ -10,15 +10,7 @@
 import { schema } from '@kbn/config-schema';
 import { i18n } from '@kbn/i18n';
 import type { ThemeVersion } from '@kbn/ui-shared-deps-npm';
-import type { UiSettingsParams } from '@kbn/core-ui-settings-common';
-
-function parseThemeTags() {
-  if (!process.env.KBN_OPTIMIZER_THEMES || process.env.KBN_OPTIMIZER_THEMES === '*') {
-    return ['v8light', 'v8dark'];
-  }
-
-  return process.env.KBN_OPTIMIZER_THEMES.split(',').map((t) => t.trim());
-}
+import { type UiSettingsParams, parseThemeTags, SUPPORTED_THEME_NAMES } from '@kbn/core-ui-settings-common';
 
 function getThemeInfo(options: GetThemeSettingsOptions) {
   if (options?.isDist ?? true) {
@@ -27,7 +19,7 @@ function getThemeInfo(options: GetThemeSettingsOptions) {
     };
   }
 
-  const themeTags = parseThemeTags();
+  const themeTags = parseThemeTags(process.env.KBN_OPTIMIZER_THEMES);
   return {
     defaultDarkMode: themeTags[0].endsWith('dark'),
   };
@@ -98,11 +90,14 @@ export const getThemeSettings = (
         defaultMessage: 'Theme',
       }),
       type: 'select',
-      options: ['amsterdam'],
+      options: SUPPORTED_THEME_NAMES,
       optionLabels: {
         amsterdam: i18n.translate('core.ui_settings.params.themeName.options.amsterdam', {
           defaultMessage: 'Amsterdam',
         }),
+        borealis: i18n.translate('core.ui_settings.params.themeName.options.borealis', {
+          defaultMessage: 'Borealis',
+        }),
       },
       value: 'amsterdam',
       readonly: Object.hasOwn(options, 'isThemeSwitcherEnabled')
@@ -111,6 +106,7 @@ export const getThemeSettings = (
       requiresPageReload: true,
       schema: schema.oneOf([
         schema.literal('amsterdam'),
+        schema.literal('borealis'),
         // Allow experimental themes
         schema.string(),
       ]),
diff --git a/packages/kbn-optimizer/src/common/index.ts b/packages/kbn-optimizer/src/common/index.ts
index 112e677c9d713..543991a92065d 100644
--- a/packages/kbn-optimizer/src/common/index.ts
+++ b/packages/kbn-optimizer/src/common/index.ts
@@ -18,7 +18,6 @@ export * from './rxjs_helpers';
 export * from './array_helpers';
 export * from './event_stream_helpers';
 export * from './parse_path';
-export * from './theme_tags';
 export * from './obj_helpers';
 export * from './hashes';
 export * from './dll_manifest';
diff --git a/packages/kbn-optimizer/src/common/theme_tags.test.ts b/packages/kbn-optimizer/src/common/theme_tags.test.ts
deleted file mode 100644
index edf58797587f6..0000000000000
--- a/packages/kbn-optimizer/src/common/theme_tags.test.ts
+++ /dev/null
@@ -1,79 +0,0 @@
-/*
- * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
- * or more contributor license agreements. Licensed under the "Elastic License
- * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
- * Public License v 1"; you may not use this file except in compliance with, at
- * your election, the "Elastic License 2.0", the "GNU Affero General Public
- * License v3.0 only", or the "Server Side Public License, v 1".
- */
-
-import { parseThemeTags } from './theme_tags';
-
-it('returns default tags when passed undefined', () => {
-  expect(parseThemeTags()).toMatchInlineSnapshot(`
-    Array [
-      "v8dark",
-      "v8light",
-    ]
-  `);
-});
-
-it('returns all tags when passed *', () => {
-  expect(parseThemeTags('*')).toMatchInlineSnapshot(`
-    Array [
-      "v8dark",
-      "v8light",
-    ]
-  `);
-});
-
-it('returns specific tag when passed a single value', () => {
-  expect(parseThemeTags('v8light')).toMatchInlineSnapshot(`
-    Array [
-      "v8light",
-    ]
-  `);
-});
-
-it('returns specific tags when passed a comma separated list', () => {
-  expect(parseThemeTags('v8light,v8dark')).toMatchInlineSnapshot(`
-    Array [
-      "v8dark",
-      "v8light",
-    ]
-  `);
-});
-
-it('returns specific tags when passed an array', () => {
-  expect(parseThemeTags(['v8light', 'v8dark'])).toMatchInlineSnapshot(`
-    Array [
-      "v8dark",
-      "v8light",
-    ]
-  `);
-});
-
-it('throws when an invalid tag is in the array', () => {
-  expect(() => parseThemeTags(['v8light', 'v7light'])).toThrowErrorMatchingInlineSnapshot(
-    `"Invalid theme tags [v7light], options: [v8dark, v8light]"`
-  );
-});
-
-it('throws when an invalid tags in comma separated list', () => {
-  expect(() => parseThemeTags('v8light ,v7light')).toThrowErrorMatchingInlineSnapshot(
-    `"Invalid theme tags [v7light], options: [v8dark, v8light]"`
-  );
-});
-
-it('returns tags in alphabetical order', () => {
-  const tags = parseThemeTags(['v8dark', 'v8light']);
-  expect(tags).toEqual(tags.slice().sort((a, b) => a.localeCompare(b)));
-});
-
-it('returns an immutable array', () => {
-  expect(() => {
-    const tags = parseThemeTags('v8light');
-    // @ts-expect-error
-    tags.push('foo');
-  }).toThrowErrorMatchingInlineSnapshot(`"Cannot add property 1, object is not extensible"`);
-});
diff --git a/packages/kbn-optimizer/src/common/theme_tags.ts b/packages/kbn-optimizer/src/common/theme_tags.ts
deleted file mode 100644
index fc126d55a4330..0000000000000
--- a/packages/kbn-optimizer/src/common/theme_tags.ts
+++ /dev/null
@@ -1,55 +0,0 @@
-/*
- * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
- * or more contributor license agreements. Licensed under the "Elastic License
- * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
- * Public License v 1"; you may not use this file except in compliance with, at
- * your election, the "Elastic License 2.0", the "GNU Affero General Public
- * License v3.0 only", or the "Server Side Public License, v 1".
- */
-
-import { ascending } from './array_helpers';
-
-const tags = (...themeTags: string[]) =>
-  Object.freeze(themeTags.sort(ascending((tag) => tag)) as ThemeTag[]);
-
-const validTag = (tag: any): tag is ThemeTag => ALL_THEMES.includes(tag);
-const isArrayOfStrings = (input: unknown): input is string[] =>
-  Array.isArray(input) && input.every((v) => typeof v === 'string');
-
-export type ThemeTags = readonly ThemeTag[];
-export type ThemeTag = 'v8light' | 'v8dark';
-export const DEFAULT_THEMES = tags('v8light', 'v8dark');
-export const ALL_THEMES = tags('v8light', 'v8dark');
-
-export function parseThemeTags(input?: any): ThemeTags {
-  if (!input) {
-    return DEFAULT_THEMES;
-  }
-
-  if (input === '*') {
-    return ALL_THEMES;
-  }
-
-  if (typeof input === 'string') {
-    input = input.split(',').map((tag) => tag.trim());
-  }
-
-  if (!isArrayOfStrings(input)) {
-    throw new Error(`Invalid theme tags, must be an array of strings`);
-  }
-
-  if (!input.length) {
-    throw new Error(
-      `Invalid theme tags, you must specify at least one of [${ALL_THEMES.join(', ')}]`
-    );
-  }
-
-  const invalidTags = input.filter((t) => !validTag(t));
-  if (invalidTags.length) {
-    throw new Error(
-      `Invalid theme tags [${invalidTags.join(', ')}], options: [${ALL_THEMES.join(', ')}]`
-    );
-  }
-
-  return tags(...input);
-}
diff --git a/packages/kbn-optimizer/src/common/worker_config.ts b/packages/kbn-optimizer/src/common/worker_config.ts
index 8881d2354740b..89a074293d998 100644
--- a/packages/kbn-optimizer/src/common/worker_config.ts
+++ b/packages/kbn-optimizer/src/common/worker_config.ts
@@ -10,7 +10,7 @@
 import Path from 'path';
 
 import { UnknownVals } from './ts_helpers';
-import { ThemeTags, parseThemeTags } from './theme_tags';
+import { ThemeTags, parseThemeTags } from '@kbn/core-ui-settings-common';
 
 export interface WorkerConfig {
   readonly repoRoot: string;
diff --git a/packages/kbn-optimizer/src/log_optimizer_state.ts b/packages/kbn-optimizer/src/log_optimizer_state.ts
index 3173ef2a05980..2bb810f45d240 100644
--- a/packages/kbn-optimizer/src/log_optimizer_state.ts
+++ b/packages/kbn-optimizer/src/log_optimizer_state.ts
@@ -10,11 +10,12 @@
 import { inspect } from 'util';
 
 import { ToolingLog } from '@kbn/tooling-log';
+import { hasNonDefaultThemeTags } from '@kbn/core-ui-settings-common';
 import { tap } from 'rxjs';
 
 import { OptimizerConfig } from './optimizer';
 import { OptimizerUpdate$ } from './run_optimizer';
-import { CompilerMsg, pipeClosure, ALL_THEMES } from './common';
+import { CompilerMsg, pipeClosure } from './common';
 
 export function logOptimizerState(log: ToolingLog, config: OptimizerConfig) {
   return pipeClosure((update$: OptimizerUpdate$) => {
@@ -80,9 +81,9 @@ export function logOptimizerState(log: ToolingLog, config: OptimizerConfig) {
               );
             }
 
-            if (config.themeTags.length !== ALL_THEMES.length) {
+            if (hasNonDefaultThemeTags(config.themeTags)) {
               log.warning(
-                `only building [${config.themeTags}] themes, customize with the KBN_OPTIMIZER_THEMES environment variable`
+                `running with non-default [${config.themeTags}] set of themes, customize with the KBN_OPTIMIZER_THEMES environment variable`
               );
             }
           }
diff --git a/packages/kbn-optimizer/src/optimizer/optimizer_config.test.ts b/packages/kbn-optimizer/src/optimizer/optimizer_config.test.ts
index a3329dcc3d57f..5fd2318953a8c 100644
--- a/packages/kbn-optimizer/src/optimizer/optimizer_config.test.ts
+++ b/packages/kbn-optimizer/src/optimizer/optimizer_config.test.ts
@@ -8,10 +8,10 @@
  */
 
 jest.mock('@kbn/repo-packages');
+jest.mock('@kbn/core-ui-settings-common');
 jest.mock('./assign_bundles_to_workers');
 jest.mock('./kibana_platform_plugins');
 jest.mock('./get_plugin_bundles');
-jest.mock('../common/theme_tags');
 jest.mock('./filter_by_id');
 jest.mock('./focus_bundles');
 jest.mock('../limits');
@@ -29,7 +29,7 @@ import { REPO_ROOT } from '@kbn/repo-info';
 import { createAbsolutePathSerializer } from '@kbn/jest-serializers';
 
 import { OptimizerConfig, ParsedOptions } from './optimizer_config';
-import { parseThemeTags } from '../common';
+import { parseThemeTags } from '@kbn/core-ui-settings-common';
 
 expect.addSnapshotSerializer(createAbsolutePathSerializer());
 
diff --git a/packages/kbn-optimizer/src/optimizer/optimizer_config.ts b/packages/kbn-optimizer/src/optimizer/optimizer_config.ts
index b09650c0708da..1b04a6fbd25a3 100644
--- a/packages/kbn-optimizer/src/optimizer/optimizer_config.ts
+++ b/packages/kbn-optimizer/src/optimizer/optimizer_config.ts
@@ -10,16 +10,9 @@
 import Path from 'path';
 import Os from 'os';
 import { getPackages, getPluginPackagesFilter, type PluginSelector } from '@kbn/repo-packages';
+import { ThemeTag, ThemeTags, parseThemeTags } from '@kbn/core-ui-settings-common';
 
-import {
-  Bundle,
-  WorkerConfig,
-  CacheableWorkerConfig,
-  ThemeTag,
-  ThemeTags,
-  parseThemeTags,
-  omit,
-} from '../common';
+import { Bundle, WorkerConfig, CacheableWorkerConfig, omit } from '../common';
 
 import { toKibanaPlatformPlugin, KibanaPlatformPlugin } from './kibana_platform_plugins';
 import { getPluginBundles } from './get_plugin_bundles';
diff --git a/packages/kbn-optimizer/src/worker/theme_loader.ts b/packages/kbn-optimizer/src/worker/theme_loader.ts
index 92a728f17f5cb..f53560692b9ba 100644
--- a/packages/kbn-optimizer/src/worker/theme_loader.ts
+++ b/packages/kbn-optimizer/src/worker/theme_loader.ts
@@ -9,12 +9,7 @@
 
 import { stringifyRequest, getOptions } from 'loader-utils';
 import webpack from 'webpack';
-import { parseThemeTags, ALL_THEMES, ThemeTag } from '../common';
-
-const getVersion = (tag: ThemeTag) => 8;
-const getIsDark = (tag: ThemeTag) => tag.includes('dark');
-const compare = (a: ThemeTag, b: ThemeTag) =>
-  (getVersion(a) === getVersion(b) ? 1 : 0) + (getIsDark(a) === getIsDark(b) ? 1 : 0);
+import { FALLBACK_THEME_TAG, parseThemeTags } from '@kbn/core-ui-settings-common';
 
 // eslint-disable-next-line import/no-default-export
 export default function (this: webpack.loader.LoaderContext) {
@@ -24,26 +19,15 @@ export default function (this: webpack.loader.LoaderContext) {
   const bundleId = options.bundleId as string;
   const themeTags = parseThemeTags(options.themeTags);
 
-  const cases = ALL_THEMES.map((tag) => {
-    if (themeTags.includes(tag)) {
-      return `
-  case '${tag}':
-    return require(${stringifyRequest(this, `${this.resourcePath}?${tag}`)});`;
-    }
-
-    const fallback = themeTags
-      .slice()
-      .sort((a, b) => compare(b, tag) - compare(a, tag))
-      .shift()!;
-
-    const message = `SASS files in [${bundleId}] were not built for theme [${tag}]. Styles were compiled using the [${fallback}] theme instead to keep Kibana somewhat usable. Please adjust the advanced settings to make use of [${themeTags}] or make sure the KBN_OPTIMIZER_THEMES environment variable includes [${tag}] in a comma separated list of themes you want to compile. You can also set it to "*" to build all themes.`;
-    return `
-  case '${tag}':
-    console.error(new Error(${JSON.stringify(message)}));
-    return require(${stringifyRequest(this, `${this.resourcePath}?${fallback}`)})`;
-  }).join('\n');
-
   return `
-switch (window.__kbnThemeTag__) {${cases}
+switch (window.__kbnThemeTag__) {
+${themeTags.map(
+  (tag) => `
+  case '${tag}':
+    return require(${stringifyRequest(this, `${this.resourcePath}?${tag}`)});`
+).join('\n')}
+  default:
+    console.error(new Error("SASS files in [${bundleId}] were not built for theme [" + window.__kbnThemeTag__ + "]. Styles were compiled using the [${FALLBACK_THEME_TAG}] theme instead to keep Kibana somewhat usable. Please adjust the advanced settings to make use of [${themeTags}] or make sure the KBN_OPTIMIZER_THEMES environment variable includes [" + window.__kbnThemeTag__ + "] in a comma-separated list of themes you want to compile. You can also set it to '*' to build all themes."));
+    return require(${stringifyRequest(this, `${this.resourcePath}?${FALLBACK_THEME_TAG}`)});
 }`;
 }
diff --git a/packages/kbn-optimizer/tsconfig.json b/packages/kbn-optimizer/tsconfig.json
index f8cb993537be7..d6e79f66a561a 100644
--- a/packages/kbn-optimizer/tsconfig.json
+++ b/packages/kbn-optimizer/tsconfig.json
@@ -18,6 +18,7 @@
   "kbn_references": [
     "@kbn/config-schema",
     "@kbn/dev-utils",
+    "@kbn/core-ui-settings-common",
     "@kbn/optimizer-webpack-helpers",
     "@kbn/std",
     "@kbn/ui-shared-deps-npm",