fix: do not reset user theme when logging out in the demo
By default, react-admin clears the local storage on logout, but the theme preference is stored there.
This MR overrides the default clear method used by react-admin to first copy the theme key and rewrite it after the clear.
See https://github.com/marmelab/react-admin/issues/7864
Closes #73 (closed)
Edited by Arnaud Vergnet