RFC: Progress bar color management

Description of the new feature / enhancement

While I'm aware that #1920 exists, that covers text which is a fairly significant area that will need a lot of thought.

On the other hand, we've got the progress bar with three existing themes. Here's where I have my questions:

1. Should this eventually be part of the same configuration effort from #1920? It should be easier to implement customizable colors for that than text, since you're generally not going to be worrying about the background color.
2. In the meantime, would PRs with alternative themes be accepted? My instincts say "probably not, because you'll want to not clog up the codebase with themes and instead make it user configurable," but that's probably a LONG ways off and very low down the priority list.

Proposed technical implementation details

Either one of:

1. Allow PRs (at least for the time being) with additional progress bar themes.
2. Start a discussion, either separate or as part of #1920, on theming of the progress bar. (I feel it probably should be separate since it's a relatively low bar to entry compared to dealing with various VT background colors..)
3. Put a hard stop on theming and say "We're not going to actually do anything in this area for a substantial amount of time; either until we've worked out how to proceed, or we decide we're not going to allow any theming outside what we already have."

Start by reading the related #1920 discussion and reviewing how the three existing progress-bar themes are handled. The issue does not identify files, tests, or a settled implementation path; completion would first require deciding whether to accept additional themes, begin a configurable theming effort, or defer the work.