LessonUp uses Montserrat as the default font. However, you can change the default font in LessonUp. By doing so, LessonUp will apply the font you have selected to all slides and text boxes (components).
Here's how you can do it:
Click on your name in the top right corner and go to "Settings."
Scroll down a bit until you find the section "Default editor settings."
Select the font that you want to use as your default font.
From now on, all texts in slides and text boxes will open in the font you have chosen.
Important to note: the font in interactive components cannot be adjusted. For these, you will need to use text boxes. You can add a text box by clicking on the "+Component" button and then selecting "Text box."