Create a Vanilla theme

We have been talking about the web team creating a Vanilla version of the starter pack. This will take some time though.

In the mean time, we should set up a temporary theme (which can later be replaced by the Vanilla theme), containing everything that is mandatory and can be put into a theme - for example, the CSS files and variables, default extensions, default extension configuration.

The advantage of that is that the theme can be updated automatically through Python, it will be easier to override/customise settings, and it will make the starter pack much smaller and with that easier to maintain.