Style is just one of many things. And it's extremely hard to properly code your own consistent UI toolkit.

On top of that every platform has a myriad of platform-specific behaviors that people expect and that you will get wrong in yours. Accessibility is the big elephant in the room. But even things like secondary focus in prompts/dialogs on MacOS (that Apple's own Catalyst and Swift forget about) is a bitch and a half to get right.