Improve css infrastructure: migrate to less and use @import

[MaxTru]

Is your feature request related to a problem? Please describe

Currently we use plain css for the styling. This (IMO) becomes a bit tedious. (Long file with only a very basic and hard to understand structure)

Describe the solution you'd like

We could improve the css struture by:

• Migrating to less
• Using @imports (e.g., to have seperate files for variables)

Describe alternatives you've considered

Keep as is

Additional context

In many other repos, we already use less and also @imports