After looking into the Theme changes, I think the new scheme is an improvement - once you go through the pain of updating any customized QSS theme you may be using. OVT files work kind of like CSS files in HTML: you can extend and modify previously defined items, rather than needing to copy everything like with QSS files.
In my case, I had tweaked the old Dark.qss to change some items in the audio fader and meter. The new Yami style "Classic" is similar to the old Dark. My attached file (with .txt added to allow posting here) extends Yami Classic, defining only a few color changes and tweaks to the fader and meter. Like custom qss files, stash this file in your OBS AppData themes subdirectory.
To create this, I looked in Yami.obt and Yami_Classic.ovt for the items I need to change (most of which I knew from my old custom qss file) and pulled the items that needed to be changed or extended. To see the effect of changes, I open "Settings" and switch back and forth between "Classic" style and my new "JClassic" style.