Hi @Audrey2023leger,
Welcome, and thank you for reaching us.
If you're not seeing this option on your device, kindly try to perform a system reboot by going to Settings > System > Power (skip if option is not available) > System Restart. Also, this support article shall guide you in modifying the theme on your device. Lastly, if theme options are limited, you can add themes manually by visiting this website here.
We hope this helps. Keep us informed of what you find out.
Sincerely,
Janadee
Nadee K.
Roku Community Moderator