Refactoring the theme CSS code

[remi-grumeau]

• Adding CSS variables for colors to make theme forks or customisation easy
• Make all sizes relative to improve responsive
• Remove weird pseudo elements positioning for top/bottom arrow
• Remove float & switch to flex display

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
New tests added?	not needed
Fixed tickets	not that i know of
License	MIT

Description

Just got my hands on medium editor, and thought this might be handy for a few people. A few more could be done but might involve breaking BC.

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Please, don't submit /dist files with your PR!