I've tried MiniCSS[1] (which styles your default HTML without adding too many classes and has themes) and MUI[2] (Material UI styles). Both are decent, but I keep falling back to Bootstrap v4 because Bootstrap just gets a lot of sizing and padding right.

With other frameworks I'm constantly adding new rules just to add a bit of padding and margin here and there, but Bootstrap 4 gets a lot of it right by default. Plus it now has a lot of useful utility classes for small padding increments (e.g. `.mb-3` for `margin-bottom: 1.5rem` and `.px-2` for horizontal padding of 1rem). With Bootstrap 4, a previous project required only 10-ish lines of global CSS[3]. Sure, it looks and feels like a Bootstrap project, but that was fine for the purpose of the app, and Bootstrap 4 looks fresh enough that I don't feel like the design is outdated.

FWIW, I've also had a lot of luck recently with Material-UI[4], a set of React components that implement the Material UI aesthetic. My current project feels like it could fit in with any Google app from G Suite.

[1]: https://minicss.org/

[2]: https://www.muicss.com/

[3]: https://github.com/statico/aspen/blob/master/pages/index.js#...

[4]: https://material-ui-next.com/