Add an example about themes

[jtpio]

There are already a few pointers to create a theme extension:

• The cookiecutter: https://github.com/jupyterlab/theme-cookiecutter
• The documentation, which points to the cookiecutter: https://jupyterlab.readthedocs.io/en/stable/developer/extension_dev.html#themes

However it would useful to have an example in this repo to show how to create a new theme. And the colors could be funky to make it fun!