So, this is the result:
This is a Beta version of the theme, so dont use in professional/personal blogs.
To install, just uncompress the new admin theme in your flatpress installation. Then, go to the admin panel, select "Maintain", and "Purge theme and templates cache".
To uninstall the admin theme, delete the "admin" folder, and upload the default "admin" folder of your flatpress version (Go to flatpress homepage, and download it).
Also, you need to reemplace this files:
Download from GitHub: https://github.com/flatpressblog/flatpr ... nsiveadmin
To see more about this theme (How to create plugins, changelog, etc), visit this link.
Edited by Arvid, 2020-09-06: Link to GitHub repo changed