add markdown-specific styles for Atom

[serverless-mom]

what

adds styling for markdown in Atom

how

Atom already adds classes to markdown dynamically, so adding some .css for those classes is all that's necessary to add colors to .md documents

concerns

• All headings have the same level and font-weight differentiates the first three levels. There's no great way (that I could think of) to continue this sequence down to h7, so h4 and lower all just a normal font weight. Also at least operator mono the bold weight is kind of ugly ¯_(ツ)_/¯

• a new background-color is added for code snippets, which hopefully will allow for syntax highlighting when Atom improves markdown support. I checked this new background with our existing font colors and it still passes the accessible colors test

acknowledgements

Ariel helped me figure this out