1. Login to MODx admin page
2. Select Tools -> Configuration -> Interface & Features
3. on ‘Path to CSS file’, enter your css path (e.g. http://YOUR_DOMAIN/web_assets/css/editor.css)
4. on ‘CSS selectors’, enter a list of selectors in this format: displayName=selectorName;displayName2=selectorName2
5. Click ‘Save’
6. Create the css file you specified in step 3
7. Add the new styles to your template css file