Equivalent to custom.css for the ACP

I've used custom.css to change the body font to a webfont. Since the flag of my locale isn't featured on the flag sprite and so isn't included in flags.css, I've amended the image and entered a custom class into custom.css. The results are exactly what they need to be:

The problem is that I would also like to use my webfont and see my flag used in the ACP. Since custom.css doesn't load for the ACP, this isn't happening for me and I get this:

I suspect this is by design, since custom.css is located under core > front in the theme designer rather than core > global.

Is there an equivalent to custom.css under core > global so that my changes will also be reflected in the ACP, or should I simply amend core > global > flags.css and core > global > framework > global.css and risk the changes getting overwritten in future updates?