Thanks for reaching out to us! In the current desktop/web version of Ribbet, this isn’t possible, however it is currently possible on our mobile app (2nd item on the Text toolbar that shows after you add some text).
In our new version of Ribbet for web, which we’re working on, this option is included. If you’re interested in becoming a beta tester of this, you can sign up to become an early, beta tester here.
One thing some Ribbet for web users currently do is add a rectangle sticker behind their text (from the Stickers > Geometric group). This can be faded to create quite a nice effect.
Or, let us know if you have any questions.
Regards – The Ribbet Team