EDIT: I recently discovered while writing up this blog post that doing this affects the following Office 2007 windows: Word, Excel, PowerPoint, Outlook, and Access. It does not noticeably affect OneNote, InfoPath, or Publisher. I could not check Groove; I did not want to make an account.
Step 1: Click the Logo
Step 2: Word Option
Step 3: Drop and Give Me Colors!
Step 4: Pick Your Poison
Then click OK.