I like Framework7 from what I have seen of it, but I have not used it. I think it comes closest to matching jQuery Mobile in intended audience and scope. It CAN be used with Vue.js or React.js, but they are not required.
I would be cautious, though, if you are building a general-use website. It was built specifically to be used on iOS and Android. It's quite specifically targeted for providing widgets for Phonegap/Cordova apps, and has native look/feel for each platform.
One design choice you have is whether you want a unified look/feel or a native look/feel. For my own purposes, we want a unified look/feel so that training material is the same regardless of platform. (I work on educational apps used in a formal classroom setting. Students might have an iOS device or an Android device.)
For something more consumer-oriented, you might like a native look/feel.
For "out of the box" Bootstrap themes, I'd suggest you explore the themes sold directly by the Bootstrap developers. Some are developed by their own developers, some by third-parties, but all curated by the Bootstrap team. The profits go to help improve Bootstrap.
I haven't used any of these, but it would be my first place to look for a theme. One thing with the themes, though, is that they might not all be up-to-date with the latest version of Bootstrap 4. It takes some time for the theme developers to get up-to-date with new Bootstrap versions.
The future of widgets is Web Components. But it is probably too soon for most projects.