Twenty Seventeen: CSS coding standards fixes.

This makes a few adjustments to CSS to bring them inline with WordPress
CSS coding standards including:

* Font weights should be defined using numeric values
* Do not pad parentheses with spaces

This also fixes a couple of typos to inline comments.

Props netweb for initial patch.
Fixes #38901.
Built from https://develop.svn.wordpress.org/trunk@39340


git-svn-id: https://core.svn.wordpress.org/trunk@39280 1a063a9b-81f0-0310-95a4-ce76da25c4cd