As this question has not really been answered yet: for the people who bought a 4/8GB Pi4: as far as this amount will never be useful for the emulators, would it be helpful to increase the VRAM to 2/4GB?
@Crispy The only change that would affect this should be the changes in Emulationstation. The desktop - I assume - is only started on-demand.
You can take a look at the theme - it's in /etc/emulationstation/themes/carbon. If you wish to modify it, copy the folder to $HOME/.emulationstation/themes/carbon-mine and make your modifications there.
@dal1980 While you're developing your theme, it's good to start ES with the --debug flag and look in the log file to spot any errors coming from the .xml files parsing. The log file is in /home/pi/.emulationstation/es_log.txt.