Chat: Improve JetBrains and Visual Studio themes (#5265)

This commit refines the JetBrains and Visual Studio themes for webviews:

JetBrains Theme:

- Removes custom scrollbar styles for pre and .model-selector-popover
elements, aligning with the default JetBrains experience (but currently
still not working correctly, will address in a follow-up).
- Uses var(--jetbrains-ActionButton-hoverBackground) for hover
background on .tw-bg-muted-transparent elements.
- Adds padding for the editor input box:

![image](https://github.com/user-attachments/assets/c6568cfb-6bd5-43f2-9b75-f12c4395d5d7)


Visual Studio Theme:

- Defines missing color variables for input validation and active
options, enhancing consistency with the Visual Studio theme. These
changes improve the overall visual fidelity and consistency of the
webview themes.
- Commented out variables that are currently not used.

## Test plan

<!-- Required. See
https://docs-legacy.sourcegraph.com/dev/background-information/testing_principles.
-->

Stylesheets update - check UI.

Currently on main:


![image](https://github.com/user-attachments/assets/b3d88b6e-3a5a-48d7-8696-c5ad1c74449b)


After:


![image](https://github.com/user-attachments/assets/7433f5c7-fd9a-43d6-b8df-eed20bf63f86)


## Changelog

<!-- OPTIONAL; info at
https://www.notion.so/sourcegraph/Writing-a-changelog-entry-dd997f411d524caabf0d8d38a24a878c
-->

lib/prompt-editor/src/BaseEditor.module.css

@@ -29,3 +29,7 @@
 .theme-paragraph {
     margin: 0;
 }
+
+html[data-ide='JetBrains'] .theme-paragraph {
+    padding: 0.5rem;
+}

vscode/webviews/themes/jetbrains.css

@@ -182,7 +182,7 @@ html[data-ide='JetBrains'] {
     --vscode-editorIndentGuide-activeBackground5: var(--jetbrains-Menu-selectionForeground);
     --vscode-editorIndentGuide-activeBackground6: var(--jetbrains-TextPane-selectionForeground);
     --vscode-editorIndentGuide-background: var(--jetbrains-Separator-foreground);
-    --vscode-editorIndentGuide-background1: var(--jetbrains-Scrollbar-foreground);
+    --vscode-editorIndentGuide-background1: var(--jetbrains-scrollbar);
     --vscode-editorIndentGuide-background2: var(--jetbrains-Scrollbar-background);
     --vscode-editorIndentGuide-background3: var(--jetbrains-Scrollbar-thumb);
     --vscode-editorIndentGuide-background4: var(--jetbrains-Scrollbar-thumbShadow);
@@ -585,15 +585,11 @@ html[data-ide='JetBrains'] {
     --vscode-editor-font-weight: var(--font-weight);
     --vscode-editor-font-size: var(--type-ramp-minus1-font-size);
 
-
     --vscode-widget-shadow: rgba(0, 0, 0, 0.36);
 
     /* todo: get this from the vscode implementation instead of hardcoded on this theme file*/
     --vscode-font-size: 13px;
-
-    /* Mimic rules injected by VSCode */
     scrollbar-color: var(--vscode-scrollbarSlider-background) var(--vscode-editor-background);
-
 }
 
 /* Mimic rules injected by VSCode */
@@ -640,38 +636,13 @@ html[data-ide='JetBrains'] pre code {
     padding: 0;
 }
 
-html[data-ide='JetBrains'] pre::-webkit-scrollbar {
-    width: 14px;
-    height: 14px;
-    background-color: transparent;
-}
-
-html[data-ide='JetBrains'] pre::-webkit-scrollbar-thumb {
-    background: rgba(255, 255, 255, 0.15);
-    border-radius: 100px;
-    border: 3px solid var(--jetbrains-Desktop-background);
-}
-
 html[data-ide='JetBrains'] blockquote {
     background: var(--vscode-textBlockQuote-background);
     border-color: var(--vscode-textBlockQuote-border);
 }
 
-
-html[data-ide='JetBrains'] .model-selector-popover::-webkit-scrollbar {
-    width: 10px;
-    height: 8px;
-    background-color: transparent;
-}
-
-html[data-ide='JetBrains'] .model-selector-popover::-webkit-scrollbar-thumb {
-    background: rgba(255, 255, 255, 0.15);
-    border-radius: 100px;
-    border: 1px solid var(--vscode-sideBar-background);
-}
-
 /* color-mix is not supported on JetBrains webview */
-html[data-ide=JetBrains] .context-item-mention-node {
+html[data-ide='JetBrains'] .context-item-mention-node {
     color: var(--vscode-inputOption-activeForeground);
     background-color: var(--vscode-badge-background);
     gap: 0.2rem;
@@ -682,10 +653,10 @@ html[data-ide=JetBrains] .context-item-mention-node {
     }
 }
 
-html[data-ide=JetBrains] .container {
+html[data-ide='JetBrains'] .container {
     background-color: var(--vscode-input-background);
 }
 
-html[data-ide=JetBrains] .hover\:tw-bg-muted-transparent:hover {
+html[data-ide='JetBrains'] .tw-bg-muted-transparent:hover {
     background-color: var(--jetbrains-ActionButton-hoverBackground);
 }