ensure that CSS4 system colors are not a fingerprinting vector

From #33534 (closed): Firefox 76 added support for CSS4 system colors. It looks like these were not added to https://searchfox.org/mozilla-central/source/widget/nsXPLookAndFeel.cpp#534 (GetStandinForNativeColor()). We should test the behavior and consider updating the system colors to Windows 10 and MacOS 10.10.x.