Add git decoration colors for tab labels

[rpbaptist]

I might open a PR for this myself, but it takes some sorting out with all the theme variances.

I see there's a number of implemented properties already, but I notice some of the tab decorations stand out since they fall back to the default colors.

This this is the full list of git decoration colors

[jdinhify]

Thanks @rpbaptist

Would you be able to open a PR? If not, I'm happy to have a look when I sort out some time.