Making a custom scrollbar because it doesn't support dark mode is worse for accesibility overall. Your custom scrollbar likely won't work well with tools disabled used for scrolling that isn't the mouse.
The confidence with which you state “is worse for accessibility overall” is just breath taking.
Had you written “may be” instead of “is” and “in many circumstances” rather than “overall” - then I would understand that you were thinking about this.
But no you’re claiming perfect knowledge of a situation that you are not inside and which I am inside.
The operating system gets it very wrong in very many circumstances. I end up having to provide - configurable - alternatives to overcome some of the egregious mistakes.
If the operating systems start to get it right- awesome I can make the overrides opt in not opt out.