diff --git a/Shoko.Server/API/WebUI/WebUIThemeProvider.cs b/Shoko.Server/API/WebUI/WebUIThemeProvider.cs
index 7f494c621..12b38ca91 100644
--- a/Shoko.Server/API/WebUI/WebUIThemeProvider.cs
+++ b/Shoko.Server/API/WebUI/WebUIThemeProvider.cs
@@ -82,57 +82,54 @@ public static bool RemoveTheme(ThemeDefinition theme)
     /// <returns>The updated theme metadata.</returns>
     public static async Task<ThemeDefinition> UpdateTheme(ThemeDefinition theme, bool preview = false)
     {
+        // Return the local theme if we don't have an update url.
         if (string.IsNullOrEmpty(theme.URL))
-        {
-            throw new ValidationException("Update URL is empty.");
-        }
+            if (preview)
+                throw new ValidationException("No update URL in existing theme definition.");
+            else
+                return theme;
 
         if (!(Uri.TryCreate(theme.URL, UriKind.Absolute, out var updateUrl) && (updateUrl.Scheme == Uri.UriSchemeHttp || updateUrl.Scheme == Uri.UriSchemeHttps)))
-        {
-            throw new ValidationException("Invalid update URL.");
-        }
+            throw new ValidationException("Invalid update URL in existing theme definition.");
 
-        using (var httpClient = new HttpClient())
-        {
-            httpClient.Timeout = TimeSpan.FromMinutes(1);
-            var response = await httpClient.GetAsync(updateUrl.AbsoluteUri);
-
-            // Check if the response was a success.
-            if (response.StatusCode != HttpStatusCode.OK)
-                throw new HttpRequestException("Failed to retrieve theme file.");
-
-            // Check if the response is using the correct content-type.
-            var contentType = response.Content.Headers.ContentType?.MediaType;
-            if (string.IsNullOrEmpty(contentType) || !AllowedMIMEs.Contains(contentType))
-                throw new HttpRequestException("Invalid content-type. Expected JSON.");
-
-            // Simple sanity check before parsing the response content.
-            var content = await response.Content.ReadAsStringAsync();
-            content = content?.Trim();
-            if (string.IsNullOrWhiteSpace(content) || content[0] != '{' || content[content.Length - 1] != '}')
-                throw new HttpRequestException("Invalid theme file format.");
-
-            // Try to parse the updated theme.
-            var updatedTheme = ThemeDefinition.FromJson(content, theme.ID, theme.FileName, preview);
-            if (updatedTheme == null)
-                throw new HttpRequestException("Failed to parse the updated theme.");
-
-            // Save the updated theme file if we're not pre-viewing.
-            if (!preview)
-            {
-                var dirPath = Path.Combine(Utils.ApplicationPath, "themes");
-                if (!Directory.Exists(dirPath))
-                    Directory.CreateDirectory(dirPath);
+        using var httpClient = new HttpClient();
+        httpClient.Timeout = TimeSpan.FromMinutes(1);
+        var response = await httpClient.GetAsync(updateUrl.AbsoluteUri);
 
-                var filePath = Path.Combine(dirPath, theme.FileName);
-                await File.WriteAllTextAsync(filePath, content);
+        // Check if the response was a success.
+        if (response.StatusCode != HttpStatusCode.OK)
+            throw new HttpRequestException("Failed to retrieve theme file.");
 
-                if (ThemeDict != null && !ThemeDict.TryAdd(theme.ID, updatedTheme))
-                    ThemeDict[theme.ID] = updatedTheme;
-            }
+        // Check if the response is using the correct content-type.
+        var contentType = response.Content.Headers.ContentType?.MediaType;
+        if (string.IsNullOrEmpty(contentType) || !AllowedMIMEs.Contains(contentType))
+            throw new HttpRequestException("Invalid content-type. Expected JSON.");
 
-            return updatedTheme;
+        // Simple sanity check before parsing the response content.
+        var content = await response.Content.ReadAsStringAsync();
+        content = content?.Trim();
+        if (string.IsNullOrWhiteSpace(content) || content[0] != '{' || content[^1] != '}')
+            throw new HttpRequestException("Invalid theme file format.");
+
+        // Try to parse the updated theme.
+        var updatedTheme = ThemeDefinition.FromJson(content, theme.ID, theme.FileName, preview) ??
+            throw new HttpRequestException("Failed to parse the updated theme.");
+
+        // Save the updated theme file if we're not pre-viewing.
+        if (!preview)
+        {
+            var dirPath = Path.Combine(Utils.ApplicationPath, "themes");
+            if (!Directory.Exists(dirPath))
+                Directory.CreateDirectory(dirPath);
+
+            var filePath = Path.Combine(dirPath, theme.FileName);
+            await File.WriteAllTextAsync(filePath, content);
+
+            if (ThemeDict != null && !ThemeDict.TryAdd(theme.ID, updatedTheme))
+                ThemeDict[theme.ID] = updatedTheme;
         }
+
+        return updatedTheme;
     }
 
     /// <summary>
@@ -165,48 +162,45 @@ public static async Task<ThemeDefinition> InstallTheme(string url, bool preview
         if (string.IsNullOrEmpty(fileName) || !FileNameRegex.IsMatch(fileName))
             throw new ValidationException("Invalid theme file name.");
 
-        using (var httpClient = new HttpClient())
-        {
-            httpClient.Timeout = TimeSpan.FromMinutes(1);
-            var response = await httpClient.GetAsync(updateUrl.AbsoluteUri);
+        using var httpClient = new HttpClient();
+        httpClient.Timeout = TimeSpan.FromMinutes(1);
+        var response = await httpClient.GetAsync(updateUrl.AbsoluteUri);
 
-            // Check if the response was a success.
-            if (response.StatusCode != HttpStatusCode.OK)
-                throw new HttpRequestException("Failed to retrieve theme file.");
+        // Check if the response was a success.
+        if (response.StatusCode != HttpStatusCode.OK)
+            throw new HttpRequestException("Failed to retrieve theme file.");
 
-            // Check if the response is using the correct content-type.
-            var contentType = response.Content.Headers.ContentType?.MediaType;
-            if (string.IsNullOrEmpty(contentType) || !AllowedMIMEs.Contains(contentType))
-                throw new HttpRequestException("Invalid content-type. Expected JSON.");
+        // Check if the response is using the correct content-type.
+        var contentType = response.Content.Headers.ContentType?.MediaType;
+        if (string.IsNullOrEmpty(contentType) || !AllowedMIMEs.Contains(contentType))
+            throw new HttpRequestException("Invalid content-type. Expected JSON.");
 
-            // Simple sanity check before parsing the response content.
-            var content = await response.Content.ReadAsStringAsync();
-            content = content?.Trim();
-            if (string.IsNullOrWhiteSpace(content) || content[0] != '{' || content[content.Length - 1] != '}')
-                throw new HttpRequestException("Invalid theme file format.");
+        // Simple sanity check before parsing the response content.
+        var content = await response.Content.ReadAsStringAsync();
+        content = content?.Trim();
+        if (string.IsNullOrWhiteSpace(content) || content[0] != '{' || content[^1] != '}')
+            throw new HttpRequestException("Invalid theme file format.");
 
-            // Try to parse the new theme.
-            var id = FileNameToID(fileName);
-            var theme = ThemeDefinition.FromJson(content, id, fileName + extName, preview);
-            if (theme == null)
-                throw new HttpRequestException("Failed to parse the new theme.");
-
-            // Save the new theme file if we're not pre-viewing.
-            if (!preview)
-            {
-                var dirPath = Path.Combine(Utils.ApplicationPath, "themes");
-                if (!Directory.Exists(dirPath))
-                    Directory.CreateDirectory(dirPath);
+        // Try to parse the new theme.
+        var id = FileNameToID(fileName);
+        var theme = ThemeDefinition.FromJson(content, id, fileName + extName, preview) ??
+            throw new HttpRequestException("Failed to parse the new theme.");
 
-                var filePath = Path.Combine(dirPath, fileName + extName);
-                await File.WriteAllTextAsync(filePath, content);
+        // Save the new theme file if we're not pre-viewing.
+        if (!preview)
+        {
+            var dirPath = Path.Combine(Utils.ApplicationPath, "themes");
+            if (!Directory.Exists(dirPath))
+                Directory.CreateDirectory(dirPath);
 
-                if (ThemeDict != null && !ThemeDict.TryAdd(id, theme))
-                    ThemeDict[id] = theme;
-            }
+            var filePath = Path.Combine(dirPath, fileName + extName);
+            await File.WriteAllTextAsync(filePath, content);
 
-            return theme;
+            if (ThemeDict != null && !ThemeDict.TryAdd(id, theme))
+                ThemeDict[id] = theme;
         }
+
+        return theme;
     }
 
     public class ThemeDefinitionInput
@@ -344,9 +338,7 @@ public ThemeDefinition(ThemeDefinitionInput input, string id, string fileName, b
         }
 
         public string ToCSS()
-        {
-            return $".theme-{ID} {{{string.Join(" ", Values.Select(pair => $" --{pair.Key}: {pair.Value};"))} }}";
-        }
+            => $".theme-{ID} {{{string.Join(" ", Values.Select(pair => $" --{pair.Key}: {pair.Value};"))} }}";
 
         internal static IReadOnlyList<ThemeDefinition> FromDirectory(string dirPath)
         {
@@ -379,7 +371,7 @@ internal static IReadOnlyList<ThemeDefinition> FromDirectory(string dirPath)
                 return null;
 
             // Safely try to read
-            string? fileContents = null;
+            string? fileContents;
             try
             {
                 fileContents = File.ReadAllText(filePath)?.Trim();
@@ -389,7 +381,7 @@ internal static IReadOnlyList<ThemeDefinition> FromDirectory(string dirPath)
                 return null;
             }
             // Simple sanity check before parsing the file contents.
-            if (string.IsNullOrWhiteSpace(fileContents) || fileContents[0] != '{' || fileContents[fileContents.Length - 1] != '}')
+            if (string.IsNullOrWhiteSpace(fileContents) || fileContents[0] != '{' || fileContents[^1] != '}')
                 return null;
 
             var id = FileNameToID(fileName);
@@ -415,17 +407,17 @@ internal static IReadOnlyList<ThemeDefinition> FromDirectory(string dirPath)
     }
 
     private static string FileNameToID(string fileName)
-    {
-        return fileName.ToLowerInvariant().Replace('_', '-');
-    }
+        => fileName.ToLowerInvariant().Replace('_', '-');
 
     private static string NameFromID(string id)
-    {
-        return string.Join(' ', id.Replace('_', '-').Replace('-', ' ').Split(' ', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries).Select(segment => segment[0..1].ToUpperInvariant() + segment[1..].ToLowerInvariant()));
-    }
+        => string.Join(
+            ' ',
+            id.Replace('_', '-')
+                .Replace('-', ' ')
+                .Split(' ', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+                .Select(segment => segment[0..1].ToUpperInvariant() + segment[1..].ToLowerInvariant())
+        );
 
     public static string ToCSS(this IEnumerable<ThemeDefinition> list)
-    {
-        return string.Join(" ", list.Select(theme => theme.ToCSS()));
-    }
+        => string.Join(" ", list.Select(theme => theme.ToCSS()));
 }
diff --git a/Shoko.Server/API/v3/Controllers/WebUIController.cs b/Shoko.Server/API/v3/Controllers/WebUIController.cs
index 7457e0141..5cb42ce2b 100644
--- a/Shoko.Server/API/v3/Controllers/WebUIController.cs
+++ b/Shoko.Server/API/v3/Controllers/WebUIController.cs
@@ -160,16 +160,42 @@ public ActionResult RemoveTheme([FromRoute] string themeID)
 
         return NoContent();
     }
+    /// <summary>
+    /// Preview the update to a theme by its ID.
+    /// </summary>
+    /// <param name="themeID">The ID of the theme to update.</param>
+    /// <returns>The preview of the updated theme.</returns>
+    [ResponseCache(Duration = 60 /* 1 minute in seconds */)]
+    [HttpGet("Theme/{themeID}/Update")]
+    public async Task<ActionResult<WebUITheme>> PreviewUpdatedTheme([FromRoute] string themeID)
+    {
+        var theme = WebUIThemeProvider.GetTheme(themeID, true);
+        if (theme == null)
+            return NotFound("A theme with the given id was not found.");
+
+        try
+        {
+            theme = await WebUIThemeProvider.UpdateTheme(theme, true);
+            return new WebUITheme(theme);
+        }
+        catch (ValidationException valEx)
+        {
+            return BadRequest(valEx.Message);
+        }
+        catch (HttpRequestException httpEx)
+        {
+            return InternalError(httpEx.Message);
+        }
+    }
 
     /// <summary>
     /// Updates a theme by its ID.
     /// </summary>
     /// <param name="themeID">The ID of the theme to update.</param>
-    /// <param name="preview">Flag indicating whether to enable preview mode.</param>
     /// <returns>The updated theme.</returns>
     [Authorize("admin")]
     [HttpPost("Theme/{themeID}/Update")]
-    public async Task<ActionResult<WebUITheme>> UpdateTheme([FromRoute] string themeID, [FromQuery] bool preview = false)
+    public async Task<ActionResult<WebUITheme>> UpdateTheme([FromRoute] string themeID)
     {
         var theme = WebUIThemeProvider.GetTheme(themeID, true);
         if (theme == null)
@@ -177,7 +203,7 @@ public async Task<ActionResult<WebUITheme>> UpdateTheme([FromRoute] string theme
 
         try
         {
-            theme = await WebUIThemeProvider.UpdateTheme(theme, preview);
+            theme = await WebUIThemeProvider.UpdateTheme(theme, false);
             return new WebUITheme(theme);
         }
         catch (ValidationException valEx)