Add docs to mention new util classes

doc/development/fe_guide/style_guide_scss.md

@@ -12,7 +12,15 @@ led by the [GitLab UI WG](https://gitlab.com/gitlab-com/www-gitlab-com/merge_req
 We have a few internal utility classes in [`common.scss`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/framework/common.scss)
 and we use [Bootstrap's Utility Classes](https://getbootstrap.com/docs/4.3/utilities/)
 
-New utility classes should be added to [`utilities.scss`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/utilities.scss).
+New utility classes should be added to [`utilities.scss`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/utilities.scss). Existing classes include:
+
+**Background color**: `.bg-variant-shade` e.g. `.bg-warning-400`  
+**Text color**: `.text-variant-shade` e.g. `.text-success-500` 
+- variant is one of 'primary', 'secondary', 'success', 'warning', 'error'
+- shade is on of the shades listed on [colors](https://design.gitlab.com/foundations/colors/)
+
+**Font size**: `.text-size` e.g. `.text-2`
+- **size** is number from 1-6 from our [Type scale](https://design.gitlab.com/foundations/typography)
 
 ### Naming