The themes included in RetroPie can be installed using the RetroPie setup script, choose Es-Theme in the RetroPie system from Emulationstation. If the theme is not available in that module, then you can copy the theme folder (distributed by the author) in one of the following locations on the Pi:
/etc/emulation/themes (needs root access and it's available through SFTP/SSH)
/home/pi/.emulationstation/themes - also accessible via file shares (Samba) at \\retropie\configs\all\emulationstation\themes