Edit libysuid-screenlock-nokia.css and change name of font to desired font. Most of the sys has css files that can be easily modified. Just serach /usr/share/themes in both blanco and base folders to find the ones that correspond to you. Place desired font in /home/user/.fonts and use System Info app to check they are properly installed.
Link them in css files like so Font: 'Desired font's name" size mm
His post is from the 22/12, there's a thread discussing/answering much of this now.
It's linked to in the Google doc guide, see the 1st link in my sig.