Help! My window background color wont change...?

For the past week, when I've gone to certain webpages my window background color has been a bright purple-blue color. This color also flashes in the window as webpages are loading.. And fonts are barely visible, usually either gray or black. This was not the case two weeks ago, so it appears I have accidentally changed it. I have already repeatedly gone to my control panel and changed the appearance/personalization options, but my windows background remains purple. (Not the desktop or the screensaver.) Any suggestions? I have Windows Vista, by the way. Thanks!