Set MCE default settings for email composer


I need to set the default font size for MCE editor, specifically when instanciated for the e-mail composer.
Followed steps described in post #11976, creating the upgrade-safe files and personalized content_css, but so far it just affects the regular ritch text fields (not the e-mail composer). What is missing for me to make the content_css to affect the e-mail composer too?

I’m using Sugar CE 6.15.20 (build 1001)