Add ability to include inline javascript (#37459)

app/javascript/inline/theme-selection.js

@@ -0,0 +1,23 @@
+(function (element) {
+  const {userTheme} = element.dataset;
+
+  const colorSchemeMediaWatcher = window.matchMedia('(prefers-color-scheme: dark)');
+  const contrastMediaWatcher = window.matchMedia('(prefers-contrast: more)');
+
+  const updateColorScheme = () => {
+    const useDarkMode = userTheme === 'system' ? colorSchemeMediaWatcher.matches : userTheme !== 'mastodon-light';
+    element.dataset.mode = useDarkMode ? 'dark' : 'light';
+  };
+
+  const updateContrast = () => {
+    const useHighContrast = userTheme === 'contrast' || contrastMediaWatcher.matches;
+
+    element.dataset.contrast = useHighContrast ? 'high' : 'default';
+  }
+
+  colorSchemeMediaWatcher.addEventListener('change', updateColorScheme);
+  contrastMediaWatcher.addEventListener('change', updateContrast);
+
+  updateColorScheme();
+  updateContrast();
+})(document.documentElement);