mail.google.com##div[aria-label="Message Body"] span[contenteditable="false"]

It is admittedly a bit beyond "easy customizability" once you find yourself sifting through element attributes in the dev console to craft a selector ("easy" shouldn't require knowing what any of those things are, I would think), but in case you still want to actually do it, this seems to work.

They have an ungodly number of event listeners. Why do they need so many? And I keep finding spans nested inside spans for no reason; i.e., the parent span only contains a child span. It's such an outrageous mess