I have a problem with Drupal: I build sites that have a protected area accessible only to site admins. I want to create a pseudo-admin role that has access to the protected area but I don't want them to access full admin/Drupal functionalities.
Which is the best thing to do in this case? Can I create a sub-admin area with styled access only to some areas (like creating content and others) without reinventing the wheel?
Yes.
Create a role, set the role permissions ( /admin/user/permissions ) to just what you need them to access.
If you need a styled theme for these users, check out sections module, you can do role and url specific themes.