[nonethewiser]

I suppose its not the most lightweight, and it's considered outdated by many but I actually think Bootstrap is great for your use case.

It uses classes to style html inline and looks pretty good. The downside is it gives a fairly generic look but I don't think you should be too worried about that.

Tailwind has largely replaced it because it offers more flexibility. You use utility classes to write the css (add padding, change color, etc.) and isnt as opinionated as bootstrap. But I dont think you want that.

If you really are going for lightweight and simple you should checkout classless css which only uses html to style things. Nothing needed from you other than including the css sheet. https://github.com/dbohdan/classless-css