Sorry for the late reply and update about the theme.
I can confirm that the latest Recalbox version have bring back an old issue where the theme backgrounds are loaded without limits until it fills the ram and shows white. On previous versions where it was fixed it was only loading previous and next systems. Maybe 3-5 I don't know. But we were able to use high resolution 1080p backgrounds. We're in 2019 and
- putting the system in 720p
- turning off samba, api and manager
are not solutions. By the way I tried 55 systems with 65kb image files and this issue still persist.
Someone at Recalbox have to fix this issue. I understand that the RPI3B+ is limited in term of ram but this is an issue that has been fixed in the past. My old backup image works flawlessly with 1080p high resolution backgrounds.
People here are also asking for the theme, just check the latest replies. It's not only me that's asking for it.
Thanks for reading me and hope this will be fixed anytime soon!