Replace the base64 with the @font-face css code from the Preference style.css and make sure those fonts are in the “fonts” folder of the theme. You will have to create a new @font-face just like the other ones except rename the font as icomoon on the @font-face css code. Get the icomoon fonts (which you can get in the theme’s download package in the folder “icons”). Just upload them to the theme’s font folder.
Anywhere there is the font declaration of “raleway” and/or open sans, you will need to replace it with the one from the Preference theme (or whichever font you decide to use).
By the way… you are actually the second person to bring up base64, so I will probably revert back to the standard @font-face in the next update of Circumference and remove it as it appears some people get nervous about it.