By default, the available font families you can use in your Quote bubble are Inter, Kreon Bold and Open Sans.
If you want to add customized font families, head over to your Settings -> Content -> Custom fonts.
First, make sure you have downloaded the font families you want to use, for example from the Google Font Library.
Then, click "Add font" at the top right of the page.
Add the font name, upload the file and you're good to go ! The font family will be available in your Branding settings.