Don't include fallback CSS properties for CSS variables. Before we
generated CSS like this:
a.tag-type-1 {
color: #c00004;
color: var(--artist-tag-color);
}
Now we generate CSS like this:
a.tag-type-1 {
color: var(--artist-tag-color);
}
This means that support for CSS variables is now required for colors to
work properly. All major browsers have supported CSS variables since
2016-2017.