I came across this thread looking for a solution to a problem similar to yours. I figured it out so I'm leaving this here in the hope that it will help someone else.
To change the default style: 1. Select the text box you want to change the style of. 2. Go to the 'Home' tab and click on the drop-down arrow below 'Styles'. 3. You should see a list of styles like 'Normal', 'Accent Text', 'Accent Text 1' etc. 4. Right-click on 'Normal' and select 'Modify' from the resulting list. 5. In the section 'Click to change' make the changes you need for your default font. 6. When you've customized as you need, click 'Ok' to save your changes.
The changes you make will affect ALL boxes that currently have the 'Normal' style applied to it. And that's it! :)