Why do we use CK-Editor? Well, obviously to create customizable contents, right? If you are creating a page, you can change the heading style, color, font-size, font-style etc with the help of CK-Editor. Well,I don't know if you could create your custom color or font-size, but changing default fonts of ckeditor is definitely possible. For that you may follow the below steps:
Step 1 : Create a css that contains the path to the fonts.