Using border-color + var() doesn't work

[bawjensen]

After recent changes such as #179, I wanted to see if it was possible to remove a hacky patch we've got on cssstyle. However, seems like it's not fully fixed, because while the parser interprets rgba(var(...)) properly as TYPES.VAR, that doesn't pass the borderColor.isValid check. However, notably backgroundColor.isValid was functioning as desired. So it seems like the crux of the issue is that the border logic doesn't use parsers.parseColor like the background logic does.

This is largely speculation, though.