Modify small change to a .js code (+css file)

Hey, I have a .js code that creates a menu on the top left side of the screen. The menu lets the users change the colors of eveyrhting on the page, and also it suppose to allow the user change fonts size (make the font bigger or smaller). The menu is working but it won't change font size on the page... I want you to add this to the .js code to make it work on the menu. Attached is the files of the project so you can see the problem (bigger and smaller font won't work). My budget for this is 10$ max.