PowToon Studio textboxes do not have a Fill option, so you need to think about the placement of text on a slide.
One alternative that you have is to insert the text into a shape, which we looked at in Chapter 6, Backgrounds, Characters, and Props. Not all shapes are available in all styles, so you might need to look around to find one that you like. If you do not find a shape that you like, you can always create an image and insert it using the technique shown in Chapter 6, Backgrounds, Characters, and Props. As usual, make sure that you choose a shape that is congruent with the overall design of your PowToon.