Hello Duncan Evans,
All administration CSS files can be found in ~/Sitefinity/Admin/Themes/Default.
So the easiest way to add your own CSS is to create a new .css file in that folder (~/Sitefinity/Admin/Themes/Default) and then import it to global1.css file with @import rule. For example write @import "YourFileName.css"; at the top of the file. So things will not get messed up after upgrades and fixes and new staff will be applied. You may only need to add the import rule in global1.css again. Specific page properties CSS is in ~/Sitefinity/Admin/Themes/Default/pages.css.
the Telerik team