You will have to search the net for EmulationStation themes, then if you get the theme package folder and copy it to /storage/.config/emulationstation/themes with SSH.
If you use windows use as a SSH client to copy the files across:
https://www.bitvise.com/ssh-client
If you use linux, you can use the command scp to copy across the files via ssh like this:
scp theme_sub_folder root@ip_address:/storage/.config/emulationstation/themes like mine would be:
scp -r es-theme-Spin-Master root@192.168.1.5:/storage/.config/emulationstation/themes
edit:
Oh yeah and default SSH password is “emuelec”.