Can i modify the default theme?

Hi, i have installed Plex and Retropie as a dual boot system, using NOOBS.
Right now i’m able to reboot from Rasplex straight into Retropie by sending the following commands through ssh:
su -c 'echo 8 > /sys/module/bcm2709/parameters/reboot_part' reboot
It would be nice if i could add an item into the secondary menu which would execute this script.
Then i would also be able to disable ssh since i don’t use it for anything else in rasplex.

Could you point me in the right direction?

Thanks in advance.