REST Countries API with color theme switcher

This is my ninth solution submitted here. This was a challenging experience because I was able to use many new techniques that I hadn't used before. I tried to always use something new to solve a problem or to accomplish a task in a different way than I had before. Only to feel challenged and expand my knowledge. Even while doing simple things that I previously took for granted, I sometimes got stuck for hours before discovering that just one line of code would solve the problem. It's always interesting to use new techniques in programming. Do you have any suggestion that you would like to make? Anything is welcome. Anyway, thanks for reading that.