@ikerr The Recalbox Team is only responsible for the default theme, which comes with Recalbox.
User-created themes are centered in the section:
https://forum.recalbox.com/category/14/themes-interface

I don't know if there is a theme that meets the requirements you want, but one of the most popular themes is:
https://forum.recalbox.com/topic/16064/recalbox-8-theme-next-pixel

It has a lot of information, it has documentation, and the creator usually answers most users' questions, check it out.

@ll2k21 I believe you are referring to pre-installed games, they are not in the same place or in the same gamelist as regular games, but there is an option in the Emulationstation menu to hide pre-installed games, please check.