If I'm not mistaken, you could just use body.wp-admin.index-php to target the dashboard.
If you want to add something else, you can use the admin_body_class filter to manipulate the classes added to the body and check what is displayed with get_current_screen:
add_filter("admin_body_class", function($classes) { $current_screen = get_current_screen(); if($current_screen->base == "dashboard") { $classes .= " dashboard"; } return $classes;});