What does the check-mark on a theme icon mean?

[UIUCKate]

I can't find this in the documentation. Does the checkmark mean the theme file hasn't been changed on the disk since it was applied in the current project? That would be useful, if it told you that. Or does it just mean that the theme is a custom one (in other words, kind of useless)?

Screenshot of theme with check-mark:

[Lilybiri]

It is the default theme, new project will use it. You can set each theme as Default theme, using the right-click menu on its thumbnail.