I finally found the way how to have custom fonts in my wiki! First, open this link: www.fontsquirrel.com/tools/webfont-generator. Second, when you're on the site, upload a font you've already downloaded, and some requirements I think should be done. Third, extract the file that has been automatically downloaded. After that, you will see a file named "stylesheet". Open that file, then copy the CSS code inside. Finally, paste the CSS code on your MediaWiki:Wikia.css page or MediaWiki:Common.css page. Purge the page, then test if it works!
Hope this helps! But if you do have a question, feel free to comment.