Css comments

[mariapaz]

I really like your design and also the theme you've chosen! How to make a simple exercise fun.

Even though the html is small, I thought it might be of interest reading about specificity in CSS and how to calculate it. As a rule of thumb you won't want to start by assigning ids.
https://webdevstudios.com/2015/05/26/css-specificity-may-not-known/

My other comment would be to take advantage of flexbox or css grid, instead of adding floats to elements. And don't just use pixels for units. Rems, ems, percentage work better for responsive design.