Detect dark theme via css variable (#17800)

* detect dark theme via css variable
* minor refactor, add documentation

If your custom theme is considered a dark theme, set the global css variable `--is-dark-theme` to `true`.
This allows gitea to adjust the Monaco code editor's theme accordingly.

docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -337,6 +337,9 @@ Community themes are listed in [gitea/awesome-gitea#themes](https://gitea.com/gi
 
 The `arc-green` theme source can be found [here](https://github.com/go-gitea/gitea/blob/main/web_src/less/themes/theme-arc-green.less).
 
+If your custom theme is considered a dark theme, set the global css variable `--is-dark-theme` to `true`.
+This allows gitea to adjust the Monaco code editor's theme accordingly.
+
 ## Customizing fonts
 
 Fonts can be customized using CSS variables:

web_src/js/utils.js

@@ -26,13 +26,8 @@ export function isObject(obj) {
 
 // returns whether a dark theme is enabled
 export function isDarkTheme() {
-  if (document.documentElement.classList.contains('theme-auto')) {
-    return window.matchMedia('(prefers-color-scheme: dark)').matches;
-  }
-  if (document.documentElement.classList.contains('theme-arc-green')) {
-    return true;
-  }
-  return false;
+  const style = window.getComputedStyle(document.documentElement);
+  return style.getPropertyValue('--is-dark-theme').trim().toLowerCase() === 'true';
 }
 
 // removes duplicate elements in an array

web_src/less/themes/theme-arc-green.less

@@ -1,6 +1,7 @@
 @import "../chroma/dark.less";
 
 :root {
+  --is-dark-theme: true;
   --color-primary: #87ab63;
   --color-primary-dark-1: #93b373;
   --color-primary-dark-2: #9fbc82;