theming: handle not being in the serverroot

Currently, the theming app assumes it's in the serverroot. However, with
Nextcloud's flexibility regarding configurable app paths, this is not a
safe assumption to make. If it happens to be an incorrect assumption,
the theming app fails to work.

Instead of relying on the serverroot, just use the path from the
AppManager and utilize relative paths for assets from there.

Fix #8462

Signed-off-by: Kyle Fazzari <kyrofa@ubuntu.com>