Where in the world app - color theme switcher

I completed this challenge, but I have identified several aspects to improve among which:

• Loading only a few countries on first load, then more when the user scrolls.
• Add tests.

One question I have is: how to properly use react-bootstrap and bootstrap with React apps?

I used those two libraries to style components and add the dropdown menu for region filters. I don't regret this choice since it allowed me to add functionality and improve the app's accessibility.

But when writing CSS, I ended up overriding rules instead of taking full advantage of the utility system where I could directly enter values for variables. Is it the right way to use react-bootstrap?