In this case Emby lives on a DietPi OS syytem, in my case i find it nicely rested on this system and easily accessed and serviced, updates etc. Plus you can run other software alongside within a dietPi system which to me is a huge Bonus.
Process : go to /opt/emby-server/system/dashboard-ui/modules/themes and uload / download/ change / modify the logos which reside in that directory.
They are logogdark.png and logowhite.png . Personally i like to add another when i have modified my logo, logonew.png so that after an update when logowhite and logodark are replaced with new emby logos i can duplictee logonew and name it logowhite, repeat and name it logodark and you personalised logos are back in place.
Workflow
Personalised Emby Branding Restore
Directory:
/opt/emby-server/system/dashboard-ui/modules/themes
Files:
logodark
logowhite
logonew
Copy files back after an Emby updates, always required.