Docs: avoid recommending global Grunt installation.

Closes #1951.

css/theme/README.md

@@ -4,7 +4,7 @@ Themes are written using Sass to keep things modular and reduce the need for rep
 
 ## Creating a Theme
 
-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 by Grunt from Sass to CSS (see the [Gruntfile](https://github.com/hakimel/reveal.js/blob/master/Gruntfile.js)) when you run `grunt css-themes`.
+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 by Grunt from Sass to CSS (see the [Gruntfile](https://github.com/hakimel/reveal.js/blob/master/Gruntfile.js)) when you run `npm run build -- css-themes`.
 
 Each theme file does four things in the following order: