Alright so I kinda found a way to force a specific style using qt5 widgets. Done by creating lazarus.exe shortcut (in taskbar or anywhere) and passing -style xxx, but so far only 'fusion' is working, does anyone know qt5 dark style names? I've looked but couldn't find anything.
Another possible way to force a style is by using stylesheets, by passing
-style default -stylesheet=path/to/style.qss
example: I put my style.qss in c:\lazarus\styles
-style default -stylesheet=styles/style.qss
But the problem with that is it doesn't match lazarus IDE, and I don't think any readily available stylesheet will since it probably must be made specifically for lazarus but I'm not sure and could be wrong but that's my experience.
EDIT: I found a way to get qt5 widget styles that are available on my system and only found ['windowsvista', 'windows', 'fusion'] so I guess I've got no option for a dark style. I might look more into it and I'll keep updating this comment with whatever I find hoping that it might help someone else moving the same path.
To find available widget styles:
python -c "from PyQt5.QtWidgets import QStyleFactory; print(QStyleFactory.keys())"
from a command prompt, obviously needs python installed. I also have PyQt5 installed with pip. I'm not sure if its output is directly involved with lazarus qt5 styling or not.