You can style the text, both within the text and the button that is shown within the component.
What you can’t do is apply more than one style to individual parts of the text from within the editor.
As a work around for the time being, you can create a single item collection, and add a rich text content field. Apply all of the styles you want in there (these will be taken from the theme manager styles you set within the editor). Then connect the single item, rich content field to the expandable text, and it keeps its styling for individual words etc.