If you want to customize your WordPress theme beyond small tweaks, you have to use a child theme.
A child theme is just a folder to keep your changes so you don’t make changes to the original (or parent) theme. The main reason to do this is so that you can update the parent theme without losing any of your customizations.
This subject is covered in a lot of different places, including our theme customization guide, but I wanted to go into more detail here.
Let’s get child themeing!