Using the Color Theme Generator application and applying a custom palette for the Windows8 theme, more specifically any of the dark themes, the RadTabControl's tab foreground colors are incorrect. Specifically, they'll be showing as dark text on a dark background - but the foreground text should be light (not dark). Restarting the Color Theme Generator application with this dark color palette as the default, you'd see the problem disappear - until you switch to a light palette (at which point you have the reverse problem).

Clearing and re-merging the theme ResourceDictionaries solves it.