Tweakcn is a great tool too. Main difference is I'm hoping ShadcnThemer will be more of a community-driven hub for sharing, starring, and forking themes - similar to how color palette websites have 1000's of user-made palettes. (I took this approach when building the Theme Studio for VS Code and it worked really well, 1000's of themes were designed and shared.)
Tweakcn also charges $ users to be able to share and save themes which I think is silly for a tool like this, should be 100% free and open source.
I also prefer the simple UX of ShadcnThemer better but I'm biased of course.
edit: would also love to be able to open preview on new tabs with middle-click.