fix: should not panic when passing system-color to color-mix (parcel-bundler#819)

Author: inottn

src/lib.rs

@@ -19311,6 +19311,14 @@ mod tests {
       ".foo { color: color-mix(in srgb, blue, currentColor); }",
       ".foo{color:color-mix(in srgb,blue,currentColor)}",
     );
+    minify_test(
+      ".foo { color: color-mix(in srgb, accentcolor, blue); }",
+      ".foo{color:color-mix(in srgb,accentcolor,blue)}",
+    );
+    minify_test(
+      ".foo { color: color-mix(in srgb, blue, accentcolor); }",
+      ".foo{color:color-mix(in srgb,blue,accentcolor)}",
+    );
 
     // regex for converting web platform tests:
     // test_computed_value\(.*?, `(.*?)`, `(.*?)`\);

src/values/color.rs

@@ -3253,7 +3253,9 @@ impl CssColor {
       + From<OKLCH>
       + Copy,
   {
-    if matches!(self, CssColor::CurrentColor) || matches!(other, CssColor::CurrentColor) {
+    if matches!(self, CssColor::CurrentColor | CssColor::System(..))
+      || matches!(other, CssColor::CurrentColor | CssColor::System(..))
+    {
       return Err(());
     }