@niente0 ,
The default Dark Mode theme has the active tab highlight as not a huge difference from the background color, but it makes more contrast than your screenshot shows, at least to my eyes:
9a989085-efbc-46bb-9fd8-6cdb0bd6f773-image.png
Maybe you’ll want to start with that theme, rather than starting with “black board” (because “black board”, and the rest of the classical darker themes, were designed with the light UI in mind; default-dark-mode is the only theme that was designed with the Dark Mode UI in mind).
But yes, as of now, the Document List “highlight” background involves interpolating between the current Settings > Style Configurator > Global Styles > Default Style > Background colour and black… so when Background colour is very dark, there isn’t much contrast (and if it’s black, there is no contrast).
So yes, your suggestion that Notepad++ look at the background color, and determine whether the “active highlight” in the Document List should go darker or lighter is a good one. We cannot do anything about changing the source code here, as a forum made up of the Community of fellow users.
There may or may not have been an issue created about that before. If you don’t know where Notepad++ issues are tracked, look in the forum’s FAQ section, which will link you there; then you can search the issues for keywords like “dark mode”, “document list”, “background”, “contrast”, and similar terms, and see if it’s been brought up before. (Might want to check active and closed issues, because maybe it’s been asked and rejected)