Arabic Google Fonts

Hi,

I want to change the font under the Font Settings. However, the custom Google Web Font that I need is not the drop down menu. How can I add it to the theme?

https://www.google.com/fonts/#ChoosePlace:select/Script:arabic

Also, If I want to modify the default font to Tahoma font, how can I do that, please?

Regards,
Salah