Fixed GroupBox foreground color issue.

If you add a ToggleSwitch (for example) to the GroupBox, it doesn't update its foreground color properly when switching themes. See attached screenshots below.

Using a light theme:
![toggleswitch_before](https://f.cloud.github.com/assets/631724/127522/ad030090-6f9d-11e2-9c97-e283bd628b6a.png)

Then switching to the dark theme:
![toggleswitch_after](https://f.cloud.github.com/assets/631724/127523/b74c3544-6f9d-11e2-92aa-128e8d4a86c3.png)

As you can see it is hard to read the text. The GroupBox template was missing a default parameter. This PR fixes that problem.