Make background properties construct-only
Make background properties construct-only

This allows us to do all the setup in the same place in the constructor.
It was effectively done there anyway, only now we can use less code and
one CSS provider instead of two.

[endlessm/eos-sdk#3886]

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib