Your widgets will get an ID based on the class name you specify.
You can then write a CSS selector based on those values. Say that you prefix your widget classes with ‘foo_’, then you can select them with:
div[id*="foo_"] {
background-color: red;
}
To add that to the admin pages add something like this to your functions.php:
add_action('admin_head', 'custom_widget_css');
function custom_widget_css() {
echo '<style>
div[id*="foo_"] {
background-color: red;
}
</style>';
}