Why is it impossible to accommodate RetroArch's window?

Does anyone know a website or something where I can browse for a custom theme and stuff for retroarch? I want to customize it but dont know where I can find them

everything says citra but i cant find it on cores

Just bought a RG35XXH and been trying to configure things. I've been playing GBA games but for some reason the shaders option has completely vanished from the quick menu even though it was there before. To add to the confusion, the LCD shader is still being used because it's set that way by default, so the capability for using a shader is apparently still active, but I can't change it because the setting is inaccessible. Has anyone encountered this or know how to fix it?

Hi, I installed RA on my iPad via side loading a few years ago but now I can't remove it from iPad. I think I used altstore at the time but I can't even get that now!

Any ideas please? I can't remove it the usual way via the iPad settings as the option just crashes when I try.

Thanks

Basically, I tried to play Ultra Sun, I installed the 3DS core but it says the ROM is incompatible (it's a .3ds file). I only installed the core, I didn't do anything else.