Go into Main and underneath "Graphics.freeze" insert
Font.default_name = "Times New Roman"
Where Times New Roman is an example, whatever font you want. Remember to include the font if you share your game though.
Also, if you wanted, you could set up a hierarchy of fonts that would depend on which fonts the player has already installed. To do so, rather than just "Times New Roman", you would list them in an array from most preferred to least preferred, so like:
["Courier New", "Times New Roman", "Arial"]
in that case it would use Courier New if the player had that font, otherwise it would use Times New Roman, and if the player doesn't have that it would use Arial.