style: Fix two issues with themed widgets in high contrast mode.

There were two issues with the existing code that we use to determine whether a
widget is styled or not.

First, it was using `color == Color::transparent()` instead of
`color.is_transparent()` to check for transparent backgrounds. That is not sound
as `Color::transparent()` is the literal value `rgba(0, 0, 0, 0)`, not the
`transparent` keyword, so the equality check would fail.

The other issue is that this function was early-returning false if that check
was returning false. It is a bug for this function to early-return false, as it
makes the result of the function dependent of the order of the declarations.

Differential Revision: https://phabricator.services.mozilla.com/D62060

Author: emilio

components/style/rule_tree/mod.rs

@@ -1458,7 +1458,6 @@ impl StrongRuleNode {
         use crate::gecko_bindings::structs::NS_AUTHOR_SPECIFIED_PADDING;
         use crate::properties::{CSSWideKeyword, LonghandId};
         use crate::properties::{PropertyDeclaration, PropertyDeclarationId};
-        use crate::values::specified::Color;
         use std::borrow::Cow;
 
         // Reset properties:
@@ -1581,11 +1580,11 @@ impl StrongRuleNode {
 
                     if is_author {
                         if !author_colors_allowed {
-                            // FIXME(emilio): this looks wrong, this should
-                            // do: if color is not transparent, then return
-                            // true, or something.
                             if let PropertyDeclaration::BackgroundColor(ref color) = *declaration {
-                                return *color == Color::transparent();
+                                if color.is_transparent() {
+                                    return true;
+                                }
+                                continue;
                             }
                         }
                         return true;