You see website such as facebook for some unknown reason has different font such as what is being used in the options because when I tried using inspect elements it always has different font because I mean its always bold but in the option and customize font I didn't use any of the font that is seen in the elements. Can someone please explain how to change the fonts?
If you can't see the font, it's because it's declared in a separate CSS file. I'd suggest using a CSS editing extension such as StyleBot. An extension like this will probably work better than editing the page via Inspect Element because with the extension, the changes will be persistent.