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

![before](/uploads/475480ff7d9326bc32f030a4762ba94d/before.png)

## After

![after](/uploads/0a49104df21ca3c52a05ac346411c034/after.png)

See merge request !12