Is it possible to use custom fonts with github's markdown? I'm currently loading an image as a logo, yet rendering a font would be faster and nicer.
rendering a font would be faster and nicer.
Yes, except you wound not use any (multiple) colors in said "icon as a font".
See "GitHub redesigns its icons using a custom font"
You can, of course, take a monotone font-rendered icon and set its colour to, for example, red or blue, but that changes the entire icon; you can’t have different parts of the icon use different colours, or use shading