By default, Windows 8 comes with a dark theme as the default, which, unfortunately, is currently buggy, as it gets locked on it. I know this because my grandma got a laptop with Windows 8, and her and my mom were trying to figure out how to get the background of a document from this dark blue-purple with white text to white with black text because they didn't like it. I actually like it, but where would I get a theme that makes the background like that? This would also apply to things like folders and such. I searched for themes online using the "Get more themes online" link when personalizing your desktop, but didn't see any that changed the contrast. Thanks in advance.
search deviantart or google for something like "windows 7 custom theme purple" until you find something you like. Once you find that follow some tutorial online. I changed my theme so long ago I don't remember what to do exactly. It's simple stuff, it only took me about 20 minutes, and that was only because I didn't follow a step correctly. You shouldn't have any problems as long as you find a reliable tutorial. I would link the one I used but I can't find it.
I've always liked CustoPack Tools. It's compatible with most any themes you'll find on deviantart or wherever, and they have a nice gallery on their site. There are other ways to install themes (as below mentioned), but Custopack integrates it all into a nice UI, it makes it all very easy to use.
