@sj TBH I don't remember how to write CSS descriptors properly 🤪
Here is the uBO rule I use to change it:
! Replace boring POST! with elephant passing wind
social.librem.one##.compose-form__publish-button-wrapper > .button.button--block:style(font-size: 0 !important)
social.librem.one##.compose-form__publish-button-wrapper > .button.button--block::after:style(content: "🐘💨"; text-transform: none; font-size: 14px)
social.librem.one is the instance I'm on
@sj Basically you just set the font size property of original element to zero then "after" style gets applied that has the "content" property set — here comes your new text.