CSS rebuild task notes

[benlk]

• mobile-first styles: media queries should be min-width:, not max-width.
• 🚫post-processing CSS into different media-query-specific files loaded with <link media="... doesn't save page load times; https://scottjehl.github.io/CSS-Download-Tests/ documents that all useragents download all styles even if the stylesheet is for a larger screen.

[benlk]

Write styleguide for LESS files

[benlk]

See Adam's suggestions on a Flexbox-related grid system in #1058.

Also: We should use CSS Custom Variables in our LESS, so that the colors cna be changed client-side.