The main thing to remember about Lit elements (web components) is that they are scoped/jailed. No CSS (apart from a few properties such as font and color and a few more) penetrates the shadow dom barrier. That makes it a bit unrealistic to use Bootstrap because it relies on global CSS classes.
You can, of course, import Bootstrap into your elements with the risk of ballooning the output with unneeded CSS.
I helped someone a while ago getting Tailwind to work, however. Tailwind should work better in the scope of shadow DOM since it is “compiled”. To make it work with Lit, you must create a wrapper component from which all your components extend.
I believe we used this guide to make it work:
(The guide also gives an idea of how to use SCSS with Lit elements)
Disclaimer: This is outside Umbraco’s scope and is solely relying on your build setup