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 proceeding: https://github.com/hakimel/reveal.js#full-setup
To create your own theme, start by duplicating a .scss file in /css/theme/source. It will be automatically compiled by Grunt from Sass to CSS (see the Gruntfile) when you run npm run build -- css-themes.
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 any selectors and styles you please.
Include /css/theme/template/theme.scss The template theme file which will generate final CSS output based on the currently defined variables.