Improve theming support
libdazzle seemingly has a mechanism to automatically extend themes, a bit like we do but per-theme and per-variant, e.g. GNOME Web uses it. We likely should copy it, I've been told to look here.
Similarly, we should implement themes with SASS to avoid duplication. SASS isn't available in runtimes so we likely should make it be compiled downstream and check it has been recompiled correctly via the CI.