Cayman is a clean, responsive theme for GitHub Pages. This theme is available as an option if you use the Automatic Page Generator or you can copy the template and styles to use on your own.
You can preview the theme at http://jasonlong.github.io/cayman-theme or with real content at http://jasonlong.github.io/geo_pattern.
If you copy this theme for your own needs, feel free to customize the colors however you like. A couple of nice sources for gradient colors are http://uigradients.com and http://jxnblk.com/shade/.
Using with Jekyll
If you'd like to use this theme with Jekyll, Pietro Menna has put together a package to make it easy. You can find it at https://github.com/pietromenna/jekyll-cayman-theme
This work is licensed under a Creative Commons Attribution 4.0 International license.