Custom CSS Code/Class

I know there is a way to target all image widgets on a page with the .widget_sp_image class, but I just want to add some CSS to a single or maybe a handful of selected widgets.
e.g. I have a transparent gif in a sidebar and I want to set the background grey, but just in that widget an no other widgets.
So it would be really helpful, if it would be possible to have a new “CSS Classes” field, for example as it is in the menu, or to have a new “Custom CSS field”. The first version is probably a bit cleaner and simpler to implement. It would really make one of the hacks that I currently have in place much cleaner, prettier and it would also have made it a bit easier.

Thanks for your great work, it is really apprecieated.

https://www.ads-software.com/plugins/image-widget/