No the theme is something that applies to all users and userroles.

If you really want this it is possible to apply some specific styling to some forms. You can say everything inside a table with a specific class1 will have all text displayed in red and everything inside another table with css class: class2 applied to it will display everything in blue. That way you can use formacces to show 2 different forms with to different layouts.

Even if you do something like this the general styling like the top, the colors of the popup titels, etc will always be the same.

I am not sure about it. However, it's possible to change the start page per user-role.