fix: cluncky nav and topbar near 60em

[sepiabrown]

I found that when the website is opened on my phone or tablet, the sidebar fills up the whole screen. Fixed that and the topbar with the hamburger button accordingly.

I have chosen 60em instead of 1024px because AFAIK -l classes such as dn-l and db-l in tachyons use 60em as their criterion for the size.