[DOCS] CSS Variables (#3015)

* docs: document css variables

* Update 02-04-css.md

docs/source/02-04-css.md

@@ -5,6 +5,28 @@ permalink: /v5/css/
 
 You can add your own stylesheet in **Admin** > **Configure** > **Advanced** > **Custom CSS**.
 
+### Use CSS Variables
+
+We recommend using CSS Variables to broadly apply changes to certain styling aspects like color or fonts.
+
+Here is a CSS example that modifies some of the CSS Variables.
+
+```css
+:root {
+  /* Change primary font */
+  --font-family-primary: 'Verdana';
+
+  /* Remove round corners */
+  --round-corners: 0px;
+}
+```
+
+You can find all CSS Variables using the Web Inspector or navigate to https://github.com/coralproject/talk/blob/master/CSS_VARIABLES.md for a comprehensive list and additional information. 
+
+> **NOTE:** Before 6.3.0 Coral uses a different set of CSS Variables. Navigate to the link above to get information on upgrading. 
+
+### Use stable CSS Class Names
+
 If you would like to change the styling of any elements of the comment embed, we provide global classnames. Most elements will be tagged with either `.coral` or `.coral-stream`.
 
 The easiest way to find the classname for the element you're looking for is to use the web inspector, and then update your stylesheet accordingly.