Notepad++ already obeys the Windows system colors, and I have been using it in dark mode by changing Windows colors. Microsoft, presumably to sabotage classical programs and to push modern UWP apps, no longer allows changing system colors. There is a Windows 10 Dark Mode instead, and all it does it notify programs about its configuration, so that they redraw all their colors. A vestige of customizable system colors that remains is the (ugly) High Contrast Mode (activated with Alt+Shift+PrtSc); but if one can put up with its ugliness, it works much better than the Windows 10 Dark Mode, changing the theme for all well-written Windows programs, including all versions of Notepad++.
With the UWP mostly dead, I was hoping that they would start caring about "legacy" programs, but nope. Notepad++, as well as many other programs I use and contribute, saw that Windows was not fixing this and started adding their own patchy dark modes, which often don't work that well. I tested the new Notepad++ dark mode, to see maybe I can start using Dark Mode instead of High Contrast Theme. Unsurprisingly, many places (settings, dropdowns etc.) remain with white backgrounds; it is difficult to change all backgrounds as it was written with obeying system colors in mind, rather than manual theming. High Contrast Theme, however, works perfectly. Presumably those remaining white patches will be fixed as Notepad++ has an active community, but dozens of other old Windows programs I use will probably never bother with explicitly adding a dark theme, so I guess I will have to stick with the High Contrast Mode.