Global header CSS is not DRY

[iandunn]

The global header styles are duplicated in many different places, which makes changes time-consuming and error-prone.

For example, adding a single menu item with a minor CSS tweak required the following commits:

• [dotorg 15504]
• https://meta.trac.wordpress.org/changeset/9247
• https://meta.trac.wordpress.org/changeset/9248

(not including the cachebust commits)

That covers 6 different stylesheets from 5 different themes, 4 of which require build processes, across two different code repositories.

When you add up the time it takes to track down where all the changes need to be made, setting up and running builds, testing, splitting up commits for different repos, doing separate cachebust commit/deploys, etc, the whole process takes a couple hours, for something that should only take 5 minutes.

I think all of the styles for the global header should be set in a single place, either style/wp4.css or themes/pub/wporg/css, rather than being spread across a growing number of themes and related files.