How to add or edit a custom class on a Component
Login to the CMS.
In the tree menu on the left-hand side, go to any page (or add a new page).
At the label 'Main content', click on an existing component (or add a new component).
In the slide-out panel, go to 'Settings' in the top-right.
At the label 'Custom classes', enter your custom class (or classes — be sure to leave a space between each class).
Click 'Submit' in the bottom right, until you return to the main page node.
Hit 'Save and publish'.
Well done #h5yr!