dom-inspector/element-picker: Ensure frame background is transparent.

This fixes the element picker on dark color-scheme websites on
Firefox Nightly and Chrome Canary at least, see:

  * https://bugzilla.mozilla.org/show_bug.cgi?id=1771047
  * w3c/csswg-drafts#4772

This improves on the existing (not quite correct, was relying on a
Chromium bug) fix for Chromium browsers.

src/js/scriptlets/epicker.js

@@ -1281,19 +1281,11 @@ const pickerCSSStyle = [
     'visibility: visible',
     'width: 100%',
     'z-index: 2147483647',
+    // https://github.com/uBlockOrigin/uBlock-issues/issues/1408
+    'color-scheme: light',
     ''
 ];
 
-// https://github.com/uBlockOrigin/uBlock-issues/issues/1408
-//   We need to reset Chromium-specific `color-scheme` property
-//   for our iframe widget.
-if (
-    CSS.supports instanceof Function &&
-    CSS.supports('color-scheme', 'initial')
-) {
-    pickerCSSStyle.push('color-scheme: initial');
-}
-
 const pickerCSS = `
 :root > [${vAPI.sessionId}] {
     ${pickerCSSStyle.join(' !important;')}