Re: That would work - but would also complicate settings even more.
I understand that adding per-theme settings would be a lot of work, and may not be worthwhile. It would only benefit people who like to switch between themes. If you do ever decide to add this, maybe you could treat it as a "theme editor", and move the related customization options out of the main settings UI.
Re: does your brain see "bold" (and brighter) subject line
The distinction between bold and non-bold is not enough for me, especially if not using the "thin fonts" option. I prefer to also use the message background and/or the line 1 color to help differentiate between read/unread. For now I just plan to use the dark theme all the time, and I will use the message backgrounds and switched backgrounds options.
I noticed another issue with the dark theme in the message list. When dividers are enabled, they aren't visible between 2 adjacent messages that have the dark background. In that case, maybe you could draw the divider line using the lighter background color?
Thanks