fix #1941: throttle scheme change

Author: tophf

src/background/color-scheme.js

@@ -126,7 +126,7 @@ function update(type, val) {
   if (isDark !== val) {
     isDark = val;
     if (isDark !== notified && saved != null)
-      debounce(notify);
+      debounce(notify, 100);
   }
 }
 

src/background/style-manager/index.js

@@ -169,7 +169,9 @@ export function getRemoteInfo(id) {
 
 /** @returns {Injection.Response} */
 export function getSectionsByUrl(url, {id, init, dark} = {}) {
-  if (dark != null)
+  // Init the scheme once, then rely on matchMedia->onchange event
+  // TODO: rework caching to set just the sender's scheme i.e. not globally
+  if (dark != null && colorScheme.isDark == null)
     colorScheme.setSystemDark(dark);
   const p = prefs.__values;
   if (init && p.disableAll) {