RGUI user here.
It’s partially customizable by editing retroarch.cfg:
menu_title_color = "FFFF00FF"
menu_entry_normal_color = "FFFFFFFF"
menu_entry_hover_color = "FFFF00FF"
video_message_color = "FFFFFFFF"
Alas, it’s missing is the (green) frame color, which is hardcoded.
Another thing and I don’t want to sound rude, but if you want this to be properly addressed by some dev, you should start doing something about that title. It sounds like “bossing” and you even used an exclamation point. I mean, there is a difference between:
“Get your free beer here NOW!”
and
“Make me a sandwich!”
I don’t mean it was your intention, but think about it .
NOTE: Color format is “AARRGGBB”, where “A” is for “Alpha”.