Not sure it this is related to the update, but after downloading that, I sampled ‘themes’ in the Scrivener menu. Not all that thrilled with any of those (they’re fine), I went back to the default theme. OK. And yes, ‘themes’ probably have nothing to do with the update.
But now when I go to compose mode, the paper is black and the print is white. I assume this must be connected to me sampling the themes, but one would expect that returning to ‘default’ would mean that compose mode would also return to ‘default’.
It’s the defaults that changed. We had been using a darker look for Composition Mode as a default, since back then there was no official Dark Mode on the Mac and so it made for a kind of simple surrogate. Now that there is a dark mode, the default composition settings were changed to match the respective mode.
So if you want just the Composition settings from Dark Mode, switch Scrivener to that configuration, and then use the Manage… button in the lower left corner of the Preferences pane to save a new theme for yourself. On the dialogue to give it a name, turn off everything except for its composition settings.
And now you have a theme that just changes Composition, not the rest. If this is a one-time thing, then I wouldn’t bother saving a preset. Instead, save the theme to a file, switch back to Light Mode, apply it from the file, and then discard the file.