Overall looks great ๐ good usage of styled-components, hooks, routing, and grommet.
I think you've gained a lot of mileage of front-end development with this app, I'm happy you were able to own and execute it!
See my review comments below, nothing too big but embracing them will help grow your mindset for future projects, most of them are food for thought...
Using CRA can add a lot of dependencies you don't need, for a cleaner usage on future projects try to use https://github.com/ShimiSun/grommet-template-prettier, or use your own webpack boilerplate that is tidy and clean.
Learn a little bit about prettier, I see you are using .prettierignore, but I don't see how you are actually using prettier, so take a look if this file can be removed if it is not being used (less is better). Prettier will help you build consistently semantic apps. https://prettier.io/ (BTW, at JSConf some people commented it looks like the HPEDev logo :) )
The project Readme file should explain a little more what this project is trying to solve, or which features it offers, I would put more details into it, people like to know what are they getting themselves into and some context will create a positive experience.
Your home page code, App.js, looks great, thanks for refactoring it.