It's recommended to modify this theme's source file by overwriting them using Hugo's theme components functionality, so the change you made remains after upgrading.
There are two empty files reserved for custom code:
To overwrite them:
layout/partials/footer/custom.html under your Hugo site folder
Insert custom code in that file.
You can modify this theme's stylesheet using the same method. Stylesheets file (SCSS) are places under
Before you do that, check CSS variables used by this theme. It's easier changing value of those variables, and less intrusive.