Add missing CSS for ".overthrow" element

Struggled getting this to work for an hour. Realized that the CSS in the readme is incomplete, and was missing a line required for ".overthrow" which is included in the demo CSS.

Added this code into the readme so it's clearer. It'd be nice to have a CSS file that's separable from the demo in the "/dist" folder, but not sure if that violates how you want this organized.