MATE shouldn't be putting the burden on every single user to have to go and fix the thing themselves just because the app chooses to behave badly.
After choosing your Light theme, you have to change the theme in Pluma as well. Now imagine that every other app is just as defective. Say the default system theme is Dark, and the user explicitly changes it to a Light one. "It's just one app", or "well you can just add Thing X to the post-install script that you don't have on a Live USB", etc). Maybe this will help clarify WHY it's so wrong, for people who don't understand why this matters (i.e. Having one app randomly be different to EVERYTHING else doesen't just look ugly and unpolished: it looks like (and is) a defect, and it's a genuine annoyance because it not only give a bad experience but it also pushes the responsibility for correcting it onto users, when it's not their fault that it's broken. Pluma's job, especially as the default editor, is to respect whatever theme the user chooses. It DOESN'T match, which is the whole point, and even if users actually want a dark theme (and sure, some do: I used one for a decade) they're still more likely to want it across the whole system, not have some apps one way and some the other. That sub-theme description is simply not true. The OOB experience is "this is Stupid and Broken and Wrong", and it is. Jep, I use Geany too (though that's partly because of an issue with VirtualBox and GFVS) - but that's not the point here, and neither is Utsuro's workaround.