Using pre-themed component libraries will seriously shoot yourself in the foot in the long run. Don't like the look of a component? Want to tweak some CSS values? Well good luck, because now you're gonna fight specificity and create a whole bunch of class rules to override already-defined rulesets. It'll blow up the complexity (and even size) of your app and significantly hurt maintainability. (As is the case with a major component library that I keep having to fight with)
Don't reinvent the wheel either by creating the components from scratch, as you will spend many hours trying to get stuff (other than looks and mouse interaction) to work (how would you interact with the components with your keyboard? How accessible is your component? Mobile usage? Etc.)
For small projects, sure, use your favorite component libraries, I'm talking about really big projects here (stuff I'm working on at work).
I suggest looking into headless component frameworks, like (in case of svelte) melt-ui, bits-ui, headless-table, and so on. All they offer are some commonly used, unstyled components where you need to provide your own CSS stylesheets and only a bit of custom logic. Using these will get you 90% of the way there. Want to have them pre-styled anyways? Shad-cn provides that, but still allows you to trim them down and restyle them the way you want.