design/mobile.css: stop adjusting font-sizes by hand

Let's see what the repurcussions are, now that we have a CSS reset.
design/mobile.css: don't use sans-serif fallback

How bad can it be to use the fonts from screen.css?
design/mobile.css: don't left-pad the sidebar
design/mobile.css: increase #sidebar size
design/screen.css: pad #main symetrically
design/mobile.css: set #main width to 95%
design/mobile.css: shrink-to-fit .codeblock
design/mobile.css: pad p, pre nicely

They're padded nicely on desktops, but not on mobiles.  Fix this.
design/mobile.css: increase .footer font-size