Enable theming via new HTML element attributes (#37477)

app/javascript/mastodon/utils/theme.ts

@@ -1,11 +1,9 @@
-export function getUserTheme() {
-  const { userTheme } = document.documentElement.dataset;
-  return userTheme;
+export function getIsSystemTheme() {
+  const { systemTheme } = document.documentElement.dataset;
+  return systemTheme === 'true';
 }
 
 export function isDarkMode() {
-  const { userTheme } = document.documentElement.dataset;
-  return userTheme === 'system'
-    ? window.matchMedia('(prefers-color-scheme: dark)').matches
-    : userTheme !== 'mastodon-light';
+  const { colorScheme } = document.documentElement.dataset;
+  return colorScheme === 'dark';
 }