Gutenberg: De-hardcode SCSS variables in our blocks

In the Markdown block (~and probably in other blocks already~ and many more), we hardcode colors, fonts and other styles that could originally are defined in Gutenberg.

Let's find a good way to manage them - also see some suggestions in https://github.com/Automattic/wp-calypso/pull/27450#discussion_r220529958

Example:

https://github.com/Automattic/wp-calypso/blob/8ad80271e0e97f627d1fafb99c74dc28ef823143/client/gutenberg/extensions/markdown/editor.scss#L1-L8

Start with the Markdown block stylesheet at client/gutenberg/extensions/markdown/editor.scss, then review the suggestions in wp-calypso PR 27450. Identify how Gutenberg defines the shared colors, fonts, and other styles before deciding how the blocks should consume them. Done means the hardcoded SCSS values are replaced consistently across the affected blocks.

Written by the indexing model from the issue text.