Bug 1733398 - Use subtler dark selection colors. r=mstange

This does the same as Android and macOS (using a semi-transparent
highlight background, and currentColor for the foreground).

This is really sort of a theming issue (where the theme
mixes system colors with theme colors).

But leaving the existing selection color is prettier
even in the dark theme, so let's do that.

Differential Revision: https://phabricator.services.mozilla.com/D127241

widget/nsXPLookAndFeel.cpp

@@ -629,19 +629,20 @@ Maybe<nscolor> nsXPLookAndFeel::GenericDarkColor(ColorID aID) {
       break;
     case ColorID::Selecteditem:  // --in-content-primary-button-background /
                                  // --in-content-item-selected
-    case ColorID::Highlight:
-      // TODO(emilio): Perhaps for selection (highlight / highlighttext) we want
-      // something more subtle like Android / macOS do.
       color = NS_RGB(0, 221, 255);
       break;
     case ColorID::Field:
     case ColorID::Buttonface:        // --in-content-button-background
     case ColorID::Selecteditemtext:  // --in-content-primary-button-text-color /
                                      // --in-content-item-selected-text
-    case ColorID::Highlighttext:
       color = NS_RGB(43, 42, 51);
       break;
-
+    case ColorID::Highlight:
+      color = NS_RGBA(0, 221, 255, 153);
+      break;
+    case ColorID::Highlighttext:
+      color = NS_SAME_AS_FOREGROUND_COLOR;
+      break;
     default:
       return Nothing();
   }