Review canvas and fix CSS

@coberholzer you're bored? good! 😄

Have a look at this repo, resp. https://canvas.confirm.ch and my CSS.
From a technical point of view, it works quite well. It also looks fine on desktop, as well as "OK" on mobile.
My "responsiveness tweaks" work (under 1200px the grid container is simply displayed as block)[canvas/canvas.css#L45-49], but the footer is a bit "all over the place" (especially when hints are switched on).

Can you have a look at it and fix it? I want dead simple CSS, no BEM and other shananigans :)

I did this within 2-3hrs, so it isn't perfect and it must not be perfect 😄