You could use a browser that supports extensions on Andriod, iOS will protect you from such nefarious things though.

iOS supports browser extensions, and has done for some time - e.g. https://apps.apple.com/us/app/makeover-custom-css/id16023611... (which demonstrates custom HN CSS in the demo screenshots!)

These are in-page CSS controls, not browser config options (no idea how those works, I avoid mobile platforms)