Custom CSS for Multisite Network Admin ?

Hello,
I am managing a Multisite network.

The My Sites drop-down menu is not wide enough for me to see the full site names, so I can’t tell them apart. The first three words are the same for each site, and that already goes off the edge of the drop-down menu. The menu needs to be wider.

I need to add the following CSS:

ul#wp-admin-bar-my-sites-list {
    width: 350px;
}

But I’ve been unable to find the correct way for customising the CSS on the Network Admin interface. It has no theme, and no customiser (obviously, as it has no front end).

Please advise how I should go about this.