This is a bit less complicated than lengths because there's no cycle
possible which could turn the color-scheme declaration invalid afaict.
So it's just that we need to defer the colors when color-scheme is
specified, which is slightly annoying, but maybe not too bad.
I had to tweak a bit the code to defer properties to fix a bug that we
were papering over accidentally. We were using the wrong registration
here:
https://searchfox.org/mozilla-central/rev/f60bb10a5fe6936f9e9f9e8a90d52c18a0ffd818/servo/components/style/custom_properties.rs#1613
That's the registration for reference.name, not for name, which
papered over some issues. The fix is simple tho, which is storing a
single CustomPropertiesMap.
Differential Revision: https://phabricator.services.mozilla.com/D211860
