Any way to set custom toolbar button images?

[domingoladron]

Summary
Is there a way to customize the images for the toolbar buttons? I don't want to add a new button, but simply change the images rendered for the buttons themselves.

I'm assuming this is either

1. a javascript 'feature' which isn't in the docs
2. a css class change (but which I can't find)

Any help would be amazing.

Version
Latest

Additional context