Add some example CSS

What does this MR do?

• add some minimalist CSS
• show an example of how to correctly reference CSS

Are there points in the code the reviewer needs to double check?

Not really...

Why was this MR needed?

The README mentions wrongly referencing CSS in Troubleshooting but does not show an example of how to do it right.

What are the relevant issue numbers?

None.

Screenshots

Before

After