[TYPO3] granting admin access to one site in a multisite

[Nagita Karunaratne]

I have created two sites on a single instance of TYPO3.

I want to give a designer access to one of the sites so they can edit the
HTML and CSS files.

Is it possible to create a user and give them permission to only administer
one instance?

I created an user and gave them access to some modules but when I login as
that user I cannot see a pagetree or any of the sites files.

What do I have to do with filemounts and DB mounts?

Or is there anything else?

Thanks,
Nagita