@ElHadjiMurad Cool! 🤩
Here is the uBO rule for the rest of us:

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: "Toot!🐘💨"; text-transform: none; font-size: 14px)

Replace social.librem.one with your instance. My instance uses fork of an ancient version of Mastodon though, I hope the class names remained the same.