The Button fonts are based on your Paragraph Styles. 

To change the Default style to one of your own, follow these steps.

1. Select your Button 

2. Open the Properties Panel

3. Click the Text Style drop-down

4. Select your Paragraph Style instead of Default

If you do not already have a Paragraph Style created for your button, you can create a new one by clicking on the Setting icon to the right of the drop-down.

Click Here to learn how to add Paragraph Styles.