Themes like User Interfaces will not be user controllable until we have secure components at all layers that users can mix and match i.e. compose. So that means no true user control over Firefox themes (or Chrome themes, or Windows themes, or any other large piece of software's themes). We basically need a Smalltalk or LISP machine-like level of composability. We'll get there. We're at the point now where we have so much cheap & fast hardware that we can re-implement LISP and Smalltalk-like composable systems at scale (i.e affordable for all like a Raspberry Pi)! Progress will happen, unfortunately it's going to take another 5 or more years!