Make the supported colorTheme LESS sections compatible with the CSS properties system

I forgot to write down the conclusion we had with Michael Hamann on the subject:

Migrating everything is a lot of work and possible bugs.
For now, we should just create a new field in the flamingo colorTheme to contain CSS customizations.

I think we should incentivize its use instead of the LESS custom code (and make sure it's easy for users to understand when they need to use LESS instead of CSS)
Down the line we'll be able to deprecate LESS customizations and hide the initial field completely, making it only accessible for backwards compatibility through the objects editor.