You can choose a green or blue theme, or you can create your own. In the css folder, there are two theme files: theme-blue.css and theme-green.css. These files have the most common CSS elements extracted in their own CSS file. Just change the hex colors to the ones you want.
In the _includes/head.html file, specify the theme file you want the output to use — for example,
theme_file: theme-green.css. See this line:
<link rel="stylesheet" href="css/theme-green.css" />
The differences between the themes is fairly minimal. The main navigation bar, sidebar, buttons, and heading colors change color. That’s about it.
In a more sophisticated theming approach, you could use Sass files to generate rules based on options set in a data file, but I kept things simple here.