Themes are written using Sass to keep things modular and reduce the need for repeated selectors across files. Make sure that you have the reveal.js development environment including the Grunt dependencies installed before proceding: https://github.com/hakimel/reveal.js#full-setup
You also need to install Ruby and then Sass (with gem install sass).
To create your own theme, start by duplicating any .scss file in /css/theme/source and adding it to the compilation list in the Gruntfile.
Each theme file does four things in the following order:
Include /css/theme/template/mixins.scss Shared utility functions.
Include /css/theme/template/settings.scss Declares a set of custom variables that the template file (step 4) expects. Can be overridden in step 3.
Override This is where you override the default theme. Either by specifying variables (see settings.scss for reference) or by adding full selectors with hardcoded styles.
Include /css/theme/template/theme.scss The template theme file which will generate final CSS output based on the currently defined variables.
When you are done, run grunt themes to compile the Sass file to CSS and you are ready to use your new theme.