Unfortunately you can’t; it would need to be implemented by the owner of the instance to make it selectable there
If you manage to create a polished theme we can have a look at integrating it. It's not that hard technically: https://join-lemmy.org/docs/en/administration/theming.html
See also: https://beehaw.org/post/537486, https://beehaw.org/post/432603
well, about that… it catches fire about once an hour and it's been interesting to try to automate problem resolution with scripts and clever rate limits. More on our uptime/SLA plans here: 📢 Buzzworthy SLA Announcement 🐝 (in !support by me)