[Glitch] Fix isDarkMode utility

Port 41639655ff to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/utils/theme.ts

@@ -5,9 +5,7 @@ export function getUserTheme() {
 
 export function isDarkMode() {
   const { userTheme } = document.documentElement.dataset;
-  return (
-    (userTheme === 'system' &&
-      window.matchMedia('(prefers-color-scheme: dark)').matches) ||
-    userTheme !== 'mastodon-light'
-  );
+  return userTheme === 'system'
+    ? window.matchMedia('(prefers-color-scheme: dark)').matches
+    : userTheme !== 'mastodon-light';
 }