style: Respect system colors in the color and background-color properties

The reason why this doesn't work is because these styles come from
datetimebox.css, which is really an author style.

We could special-case these elements, but the approach that the CSSWG resolved
on for the new forced-colors spec is to respect system colors specified by
authors, see:

https://drafts.csswg.org/css-color-adjust-1/#forced-colors-properties

So this moves us towards that, and fixes the issue nicely.

Differential Revision: https://phabricator.services.mozilla.com/D108321

components/style/properties/cascade.rs

@@ -416,6 +416,10 @@ fn tweak_when_ignoring_colors(
     // A few special-cases ahead.
     match **declaration {
         PropertyDeclaration::BackgroundColor(ref color) => {
+            // We honor system colors.
+            if color.is_system() {
+                return;
+            }
             // For background-color, we revert or initial-with-preserved-alpha
             // otherwise, this is needed to preserve semi-transparent
             // backgrounds.
@@ -433,7 +437,10 @@ fn tweak_when_ignoring_colors(
             }
         },
         PropertyDeclaration::Color(ref color) => {
-            // We honor color: transparent, and "revert-or-initial" otherwise.
+            // We honor color: transparent and system colors.
+            if color.0.is_system() {
+                return;
+            }
             if alpha_channel(&color.0, context) == 0 {
                 return;
             }

components/style/values/specified/color.rs

@@ -561,6 +561,11 @@ fn parse_hash_color(value: &[u8]) -> Result<RGBA, ()> {
 }
 
 impl Color {
+    /// Returns whether this color is a system color.
+    pub fn is_system(&self) -> bool {
+        matches!(self, Color::System(..))
+    }
+
     /// Returns currentcolor value.
     #[inline]
     pub fn currentcolor() -> Color {