From Xojo Documentation
Name of the font used to display the caption.
You can enter any installed font or the names of the two metafonts, "System" and "SmallSystem".
The System font is the font used by the system software as its default font. Different operating systems use different default fonts. If the system software supports both a large and small System font, you can also specify the "SmallSystem" font as your FontName.
On macOS, "SmallSystem" specifies the smaller system font and may make the control smaller in size as well. On Windows and Linux, "SmallSystem" is the same as "System".
This code (in any BevelButton event) sets the FontName property.