Honestly, these days you'd be better off learning Tailwind CSS[1]. I previously used Bootstrap, but Tailwind CSS is so much nicer.
I prefer Tailwind CSS over Bootstrap because it is a utility-first framework, allowing me to style elements directly with low-level utility classes, rather than relying on the predefined styles and components of the framework.
The Tailwind folks have done a good job making it highly customisable. Ultimately, it comes down to personal preference and needs.