Use absolute units with background-position
Created by: jrob-io
Fixes #324 (closed).
1rem will normally convert to 16px, but it's a relative unit that is based on the font size of <html>. If the user's browser overrides the default font size, it'll change the value of 1rem and affect how the icons are taken from the spritesheet. Switching to px when using background-position will prevent this.