DEV: Add link missing from CSS theme developer tutorial (#54)

Author: martin-brennan

docs/07-theme-developer-tutorial/03-css.md

@@ -6,7 +6,7 @@ id: theme-developer-tutorial-css
 
 Technically, Discourse uses [SCSS](https://sass-lang.com/) to author its stylesheets. However, we're increasingly moving towards native CSS features as they mature, so the vast majority of themes won't need to use SCSS features or syntaxes. We follow a [variant of BEM for our CSS](https://meta.discourse.org/t/361851), you should be familiar with this when writing theme CSS.
 
-This chapter will focus on Discourse-specific subjects, so if you don't already have a passing familiarity with CSS, take some time to learn about it from
+This chapter will focus on Discourse-specific subjects, so if you don't already have a passing familiarity with CSS, take some time to learn about it from https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Styling_basics.
 
 ## Authoring theme CSS