Instead of listing all the drive letters, you could have a single line for *:\.
But doing this will also mean losing the special icons some folders have, which isn't ideal.
Changing the system-wide default folder icon is probably better, although I'm not sure how you do that in Windows these days. The "obvious" registry changes I found didn't seem to work, but there must still be a way to do it.
As an aside, if you only want to change the icons of individual folders, using the standard Properties dialog is the way to do that, since it will work outside of Opus as well.
Thank you Darren and Leo.
Please tell me, what is the reason there is a page for Toolbars > Icons? I assume from your answers that this is only for Toolbars and not for customising the appearance of folders at all, yes?