Might there be any risk that other themes may not work well with the changed icon references? I guess this is hard to answer; the icon loading mechanism are big mystique to me and so I'm just wondering how much we could break with this change.
Well, I'm not 100% sure, but I think yes, there is a small risk: IIUC GTK falls back to and from symbolic icons, but *only if the requested version does not exist in the theme **at any size*** -- hence the bug we have, and it disappearing if you remove the offending icons. But this also means that a theme that has symbolic icons but only at some sizes that don't fit our needs would start breaking if we ask for the symbolic version.
I'll make some experiments though, because icon fallback rules is a tiny bit mystique to me as well.