I've discovered this accidentally. You can change the title if you use the pango markup language.
Right-click on the Menu then Properties.

When the whisker menu settings dialog  opens, set 'Display:' to 'Title' or 'Icon and title'. That way text will be displayed on a Whisker Menu button.

Then, change the 'Title' to this:

<span foreground="black" size="large">Menu</span>

foreground defines the font color
You can set your own font color. For example red, green, #fff, #fcfc02...
size defines the font size, some of them are 'xx-small', 'x-small', 'small', 'medium', 'large', 'x-large', 'xx-large'
If you want to see more options you can use to follow this link

P.S. I've contacted the developer and he will not be adding options to control the pango markup.