I already did that, and it did nothing.
it must've done 'something'. what's the output from the terminal? are the assets installed but not working, or do they refuse to install at all?
if they're installed but not working, have a look at your config files (/etc/retroarch.cfg and ~/.config/retroarch/retroarch.cfg) and make sure that "assets_directory" points to a directory where the assets are actually located.
alternatively, there's an updater included within RetroArch that should be able to install necessary files without using system package management.
it's disabled by default, so you'll have to set 'menu_show_core_updater = "true"' in your config, then look here
and check the "Download Art Thumbnails for All of Your Games" section.