LocalStorage is per host though. You can't track people using LocalStorage, right?

LocalStorage is per client, per host. You generally can't track people using LocalStorage without some server or database on the other side to synchronize the different client hosts.

GDPR rules are around personal preference tracking, tracking, not site settings (though it's grey whether a theme preference is a personal one or a site one).

	▲	root_axis 2 days ago \| parent [-]
		> though it's grey whether a theme preference is a personal one or a site one In this case it's not grey since the information stored can't possibly be used to identify particular users or sessions.