Link in Themes documentation is dead

[stefanocoretta]

revealjs.com/src/themes.md

Line 36 in 88ae6b3

All theme variables are exposed as CSS custom properties in the pseudo-class `:root`. See [the list of exposed variables](https://github.com/hakimel/reveal.js/blob/master/css/theme/template/exposer.scss).

Hi! The link in the Themes page seems to be dead.