Based on this post,
Change the entire font style of all dialogs
How to change the font text of the entire installer in Graphical Installer?
When you create the installer using this tool, it creates two Inno Setup files. One file the main installer file and the other has the code to set the theme. This second file has the extension ".graphics.iss". It has the following code,
#define public ButtonNormalColor "$FFFFFF"
#define public ButtonFocusedColor "$FFFFFF"
#define public ButtonPressedColor "$FFFFFF"
#define public ButtonDisabledColor "$FFFFFF"
Adding this code,
#define public ButtonFont "Times New Roman"
does not change the font.