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 installed before proceeding: https://revealjs.revealjs.com/installation/#full-setup
To create your own theme, start by duplicating a ```.scss``` file in [/css/theme/source](https://github.com/hakimel/reveal.js/blob/master/css/theme/source). It will be automatically compiled from Sass to CSS (see the [gulpfile](https://github.com/hakimel/reveal.js/blob/master/gulpfile.js)) when you run `npm run build -- css-themes`.
This is where you override the default theme. Either by specifying variables (see [settings.scss](https://github.com/hakimel/reveal.js/blob/master/css/theme/template/settings.scss) for reference) or by adding any selectors and styles you please.