scss structure: imports => theme files need to import the base class

we will need to adjust the structure:

theme files contain the scss vars as well as the css vars.

we need to break up this structure into 2 files.

example: to build a split button file, we are using the scss vars of button.Base. the output must not contain the css vars for button.Base.

For a build we need: all parent hierarchy files for the scss vars, the current level of the css vars, mixins.

src files do only need to import the mixins.