add colorScheme.isDark()

background/color-scheme.js

@@ -34,13 +34,12 @@ const colorScheme = (() => {
     onChange(listener) {
       changeListeners.add(listener);
     },
-    /** @param {StyleObj | 'darkUI'} val - the string is used by the built-in dark themer */
-    shouldIncludeStyle(val) {
-      return val === 'darkUI'
-        ? isDarkNow
-        : prefs.get(kSTATE) === 'never' ||
-          !SCHEMES.includes(val = val.preferScheme) ||
-          isDarkNow === (val === 'dark');
+    isDark: () => isDarkNow,
+    /** @param {StyleObj} _ */
+    shouldIncludeStyle({preferScheme: ps}) {
+      return prefs.get(kSTATE) === 'never' ||
+        !SCHEMES.includes(ps) ||
+        isDarkNow === (ps === 'dark');
     },
     updateSystemPreferDark(val) {
       update('system', val);

js/dark-themer.js

@@ -11,11 +11,10 @@
  * and not even a problem in the most popular case of using system dark/light mode.
  */
 
-API.colorScheme.shouldIncludeStyle('darkUI').then(val => {
+API.colorScheme.isDark().then(isDark => {
   const ON = 'screen';
   const OFF = 'not all';
   const map = {[ON]: true, [OFF]: false};
-  let isDark = val;
   toggleDarkStyles();
   msg.onExtension(e => {
     if (e.method === 'colorScheme') {