I made this application as I didn't like how the new Nextcloud design looked. I started using the Custom CSS app to change it but after a few more modifications and asks from friends I felt the need to make it into an application for it to be easy to update, and for others to find it easier.
It is a simple app that only does one thing: it injects the bundled-in CSS code into Nextcloud, and the CSS is what matters.
For the CSS we used variables that automatically changed all "pill"-style buttons "border-radius" value to a less rounded one, and some code to fix the ones that became broken in the process.
To be updated at a later time
There is a bash script for packaging the application included in the 'code' directory. It makes a tar file using the required file structure, and can optionally sign it.
The script doesn't require root permissions, just a few packages: git, xmlstarlet, tar