Unfortunately, the way X_ON_D-Y icons are done doesn't allow for that (at least, in apps I've been using). The reason is, the "X" part and the "_ON_D-Y" part are technically different glyphs, one laid on top of the other, but they can only overlap if they have the same text style. Otherwise, they're treated as two separate bits of text that just happen to come one after the other.
(And because they overlap, but have to have the same colour, the X_ON_D-Y icons in Dark fonts look like they're just dice shapes.)
One possible future development for Dicier could help with this, but it has its own drawbacks. I'm gonna post an overdue devlog/blog post about this soon!