Now, a developers of google scripts created a apps to change the sites fonts. This Fonts changer apps has more than 500 styles with a great collection.
You can also use your custom fonts, you just have to upload your fonts in .ttf, woff, .otf format and use that fonts for a specific website or all websites.
You can change font for all sites or for a specific sites such in, Facebook, twitter or etc. with Google Web Fonts.
How to ?
1. First, download and install this Chrome extension in your chrome browser.
2. Now, After installation, a small icon be appear inside the toolbar.
3. Now click on the icon, and then select your fonts and click on done.