Use CSS Variables for default Theme(s)

## Is your feature request related to a problem?
**Please describe.**
To make overruling styling of a Theme easier, we should use CSS variables for some of the styling

## Describe the solution you'd like
Analyze what variables are useful and add them.

## Describe alternatives you've considered
The current Templates are all Less based, we could also stay with that, but CSS variables seems like an easier option for minor changes.

## Additional context
We decided not to implement this for v 08.00.00 to mot broaden the scope of that release even more..