Allow users to theme the UI's colours

Hi.

It'd be great if users could costomers the UI's colours for different types of stack frame or component (eg in thread view but also in the flamegraph) then users could pick colours that work for them (in meaning and contrast). If users can choose the background & forground colours then we can pick even better colours for the frames/graphs.

In the presentation today the speaker (sounded like Greg's voice) said sorry for the yellow JS frames. but Yellow contrasts poorly with the background.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-234)