feat: add global server custom CSS (#6753)

* feat: add global server custom CSS

* refactor: reuse CodeMirror for CSS editors

apps/docs/docs/advanced/styling/index.mdx

@@ -85,8 +85,11 @@ To do this, enter the board settings:
 
 ![](./img/enter-board-settings.png)
 
-Scroll to the **Custom CSS** section on the board settings page. It is part of the unified settings form; enter your CSS
-there and save the board settings to apply it to that board.
+Administrators can add instance-wide CSS under **Management → Settings → Branding → Custom CSS**. Enter CSS directly
+or import a community theme from the Workshop, then save the server settings to apply it to every page.
+
+For board-specific styling, scroll to the **Custom CSS** section on the board settings page. Enter your CSS there and
+save the board settings to apply it to that board. Board CSS is loaded after the global stylesheet and can override it.
 
 ![](./img/custom-css-setting.png)
 

apps/docs/docs/management/settings/index.mdx

@@ -142,6 +142,10 @@ Branding settings let administrators adapt Homarr to the identity and visual sty
 
 Choose the primary and secondary colors used by the theme. **Lock primary color** prevents users from overriding the configured primary color. **Default corner style** controls the global radius used by cards, controls, and other interface elements.
 
+### Global custom CSS
+
+The branding settings include a CSS editor for styles that should apply across the entire instance. You can write CSS directly or import a community theme from the Workshop. CSS configured on an individual board takes precedence over the global stylesheet.
+
 ### Sign-in page
 
 You can set a greeting and a background image for authentication pages. The overlay slider adjusts the contrast between the background and the sign-in content. The visibility controls independently show or hide the application name, logo, and greeting.