I wouldn't recommend trying to modify DOM elements via CSS or extensions if you can avoid it. You're going to run into some broken web experiences.
Regarding scrollbars, you can set your OS to always show them. As a web dev, I'm out here trying to advocate to never screw with scrollbars.
I've explicitly made that point in the example Codepen showing that styling:
https://codepen.io/dredmorbius/full/QwXBEO
More generally, I've long held that Web design as practiced isn't the solution, it's the problem.
https://old.reddit.com/r/dredmorbius/comments/256lxu/tabbed_...
I've been toying with the notion of an FYWD browser, standing for "Fine Young Western Dinosaurs" or "Flip Your Web Design", where some of the letters in "flip" were originally different.
If I'm resorting to extensions or CSS to modify DOM elements, it's likely the web experience is already broken from my point of view. It's not something I do just for the fun of it.
And I actually think that begin able to modify webpage to your liking is important, even if it means breaking the page in the process (I'm kind of used to it with umatrix)
I do agree that in an ideal world, every webpage would be an amazing web experience, but unfortunately that's not the case, even with big websites with amazing UX desginer
You can also design the page to endlessly ask for new content as one scrolls, but never display a scrollbar...