Add doc for custom themes
Add doc for custom themes

Mostly just c+ped from Bruno's PR: https://github.com/matrix-org/matrix-react-sdk/pull/3503

vector-im/riot-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/riot-web,vector-im/riot-web,vector-im/riot-web,vector-im/riot-web,vector-im/riot-web