A lot of Theme Developers already include ways of supporting them through their Github Repository (in the Readme and/or Sidebar) and some, off the top of my head @kepano, include a link at the bottom of their Style Settings. Personally I feel like the monetisation portion of Theme Development is a little tricky since no one wants to come off as though they are expecting users to pay them for their work. Would it be nice? Sure. As mentioned it takes a lot of time to maintain themes, but by no means should it be expected in my opinion. I feel like for most Theme Developers it’s just a way to give back to a great community and to have a project to work away at.
If the Community Theme Store gets reworked, then an option to add a donation link for your theme could be an option. But that is entirely up to Obsidian developers.
With a stronger push for monetary support, there would be greater expectations out of Theme Developers and not all have dedicated time to work on their themes but only portions of the day/week to chip away at it.
As for splitting donations between Theme Developers and Obsidian Developers, I personally don’t feel like the amount of money being donated at the moment is significant enough for it to be something that is worth considered at the moment. Also there are already multiple ways of supporting the development of Obsidian directly with Sync, Publish, Catalyst, Commercial Licences, etc.