You do realize that you see a browser page presented by a third party program (vulgo: your web browser). So it will happen to you for other web pages with a bright background as well.
software has new dark theme >>> user chooses dark theme >>> users expects to see some kind of dark theme applied also to help files >>> user is disappointed to see that the software did not choose to open alternate version of help files >>> user does not check for their hypothetical existence so to not waste time
If you and more importantly Mr. @Florian do not follow that path of thought, then I will have to lobotomize my brain, so that it will maybe stop producing such logical expectations of aesthetic nature
What kind of integration do you speak of?
I already have a dark theme in my Firefox and I am using Stylish with some Themes for some sites
But the problem with Stylish is that the Themes exist only for popular and mostly international sites and often do not cover all the features / colors on a given site or [at least in my opinion do it somewhat badly]. And also for example after trying out many different ones I am using concurrently two different Themes for YouTube, thus having an overlapping end effect. But when YouTube will make profound changes then once again I will have to look for proper Themes, as the current ones might break. And for APOD, which I visit almost everyday, I had to write me a Theme from scratch- it took me a lot of time and still I could not change one annoying element
And so- I am suppose to write another Theme just for help files of Mp3tag?
The help files use a browser window. So my suggestion was to have the browser developer implement a dark mode to support your ask. I can’t think mp3tag should rewrite all of the OS integration modules. This would be a fairly monumental task I would think, and certainly not efficient for @Florian to develop.
You describe your own themes that you have put together, there is likely some conflict there but not something that mp3tag should have to deal with in my opinion.
Because the data content was re-written - and is now available on-line; thus by using a Stylish theme that I already have for this forum, by extension its colors are applied to the help files