UI component libraries for Svelte are a little lacking in maturity in my experience. I would recommend including a CSS library such as Bootstrap or Materialize. Tailwind is also popular, but not my cup of tea.
There are good alternatives to React that are as actively developed. There don't seem to be good alternatives to Bootstrap that are nearly as actively developed (excluding CSS utility class libraries like Tailwind).
My team recently evaluated 30-40 CSS frameworks to use with a custom component library, and we were surprised when Bootstrap was the clear winner. Other CSS frameworks have gotten buzz over the years, and we assumed something new and shiny would come out on top, but when we evaluated all of them, we saw sporadic commits, many unresolved issues (e.g., see Bulma issue 3074), small bus factors, inadequate support for keyboard navigation, and other issues.
So using Bootstrap with a newer alternative to React is valid. (Though, we're using Bootstrap with Next.js.)
By all means, stick to your React or Java backend or whatever it is you do if that's where you feel most comfortable. No need to feel so threatened by new alternatives.
