Customisation done using f_theme is not retained after a reboot.
Would be a bit tricky to implement as an officially supported feature since framebufferphone doesn't run any arbitrary code in the user's shell environment. The zig fbp daemon starts and is essentially separate from the user's login shell.
The user could ofcourse add stuff to their ~/.ashrc and maybe hook into f_theme via implementing & calling a function (in f_theme) that saves/restores state to a filesystem file.