Not OP, but I recommend using plain css. With the grid layout and flexbox it has become pretty easy to create something decent and learning flexbox and grid isn't hard. Grid works basically the same as Bootstrap: split a page in parts and tell each element how many parts they can use. But with gird you don't to split your page in 12 part, but can choose for yourself how many you want. Flexbox is rid, but for things you want to arrange 1d instead of 2d.

However, if you know how to work with Bootstrap, I don't see a reason to learn something else for side projects, if you want to minimize the effort you put into front end stuff. It might not be ideal, but it's good enough imo.

Plain CSS is missing the main feature of these libs, which is simple-as-crud components that match visually and help you build out pages that have the UI patterns users expect.

I second using vanilla css. Vanilla css is a lot easier to understand than bootstrap imo. Vanilla css + HTML5 is good enough for pretty much any web page.

Yup. Learning basic web platform technologies is the best investment too. They're not going away, ever. (Comparend to the currently fancy libraries/frameworks). And it will help you understand all the things built on top of the web platform.

At most use the Bootstrap grid system. Roll your own component styles, particularly if you're trying to sell whatever you're building as a product. Otherwise, your project will lack identity. I'm more sensitive to it than most because of my background, but if I see an app using Bootstrap defaults, it negatively affects my opinion.

That said, the Bootstrap 4 defaults are much more modern looking than Bootstrap 3's.

Try Tachyons. I’m playing with it for a new project and liking it a lot so far.

I like to start with Skeleton CSS or similar, strip out all the column stuff, and just rely on grid and flexbox to build the layout.

Bulma is great for starting :)