new css possibilities

[astroidex]

Describe your feature request

• https://doc.mapbender.org/en/customization/css.html
• https://docs.mapbender.org/3.0.6/en/book/css.html

Varibales

• style via css-variables in css editor or bundle scss-file
• https://github.com/mapbender/mapbender/blob/master/src/Mapbender/CoreBundle/Resources/public/sass/libs/_variables.scss
• https://github.com/mapbender/mapbender/blob/master/src/Mapbender/CoreBundle/Resources/public/sass/template/fullscreen_variables.scss
• https://github.com/mapbender/mapbender-workshop/blob/master/src/Workshop/DemoBundle/Resources/public/demo_variables_blue.scss