Ok, well I figured out something that works though I'm not sure this is how I'm supposed to do it. I noticed a new file /src/AppHelper.php and from that gathered I needed to add to the top of my code:

use Friendica\AppHelper

It seems if you include a use that doesn't actually exist it just ignores it, so that didn't throw any errors in the stable 2024 version.

Then later in my code I did:

$fv = explode('.',App::VERSION);
if ( $fv[0] == '2024'){
     $current_theme = DI::app()->getCurrentTheme();
} else {
     $current_theme = DI::appHelper()->getCurrentTheme();
}

The add-on works in both stable 2024 and the 2025-rc now. But is that a good way to get the Friendica version number?

I think art4, who wilhelm just tagged, is the best qualified to answer that, as he made the relevant PRs that refactored addons.

I guess this one should be updated:
wiki.friendi.ca/docs/addons
...except from what I've read, that wiki is frequently pretty out of date, so maybe it'll be replaced by something else instead. Incidentally art is also working on some greatly needed updates to how documentation is handled, but that may be more user centric documentation rather than developer documentation? Not entirely sure. Either way, important work.