[Glitch] Fix inversion of emoji colours based on dark/light mode

Port 591776d7ad to glitch-soc

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

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

@@ -0,0 +1,13 @@
+export function getUserTheme() {
+  const { userTheme } = document.documentElement.dataset;
+  return userTheme;
+}
+
+export function isDarkMode() {
+  const { userTheme } = document.documentElement.dataset;
+  return (
+    (userTheme === 'system' &&
+      window.matchMedia('(prefers-color-scheme: dark)').matches) ||
+    userTheme !== 'mastodon-light'
+  );
+}