Customizing admin look and feel for just one particular type of user

Looks like an almost duplicate of Back End Interface Plugin.

In toscho’s Answer, you’d only need to add a verification for your user’s role:

if( current_user_can( 'custom_capability' ) )
{
    $settings = new stdClass;
    // etc
}

The plugin MP6, which explores the future core interface, uses a similar approach and enforces the Admin Color Scheme with the filter get_user_option_admin_color.

And, I just saw that MP6 uses bits (not sure how much) of toscho’s Clean Admin, another cool theme for the dashboard.

Just like the first example, both plugins can be adapted to use a conditional admin theme based on the user role.