Most of us are content with the default theme shipped with Google Chrome- it blends in well with the default Windows and Mac visual styles. Occasionally, though, Chrome can stick out like a sore thumb (like when you switch to an obscure GTK theme on your Linux box).
Sometimes, too, you just want to shake things up. Chrome's Web Store has plenty of themes, but if you're looking for something specific, it can be a needle in a haystack proposition. If you're underwhelmed by the selection, it's quite simple to make your own theme, using your own colors and images.
Google themselves released a tool to create custom themes, but it's pretty basic. Theme Beta is a site purely dedicated to hosting various Chrome themes, but they also created a theming tool that gives you a little more power than Google's extension, and it's just as easy to use- just head to the site, add your own images or colors, and the live preview will show you just how your theme will look once applied. Play around until you get it just right, then you'll have the option to download as a .crx file (which will install directly in Chrome), package the theme and download it to your PC, or, for the pixel aficianados who want to dig even deeper, download the package as a .zip file to dig into the internals of the theme like the tint on the buttons.
Notably, ThemeBeta's Theme Creator is available on the Web Store as an extension, but it's not necessary to install it- the web app will do just fine (the extension is simply a link to the web app, anyway).
Happy tweaking. (Quick pro tip: if you want a flat look without the gradients, use an image to get the desired effect. Using colors alone will result in what, to me, is a fairly drastic gradient effect.)