Here I leave you my first custom theme, based on simplelight for Recalbox 4.1.
I have modified some images, sounds and logos.
In addition I have enlarged the size of the screenshot of each game, since I saw it very small, editing the xml of each system.
PREVIEW VIDEO YOUTUBE: https://www.youtube.com/watch?v=jJLr9HyVlc8
I hope you like it, I leave here a link for download in the video's description
You just need to unzip and copy the folder in the path /share/system/.emulationstation/themes