@emwhy: Yeah, that would be nice, as an option.
Here's a status update:
The backgrounds are generally a little lighter. I find it works better to sacrifice a bit of darkness in exchange for a bit less contrast - since some elements are hard-coded bright in Reaper.
Midi editor:
Mostly finished, but I've yet to find a way to alter the actual notes themselves when using coloring from track or item, which is what I use.
The MCP is still untouched. I never use it, but I might do a reskin of that too - if only to be able to provide a complete Affinity Designer document for theming.