Dark Mode: split color variables to top of shared_vars_css.html

This allows us to set light/dark mode variants at the top and reuse them
throughout the file (and elsewhere, in some cases). I also made use of
some of the variables in some cr_elements (i.e. toolbar, menu, etc.).

I also start looking at some of the pages with --force-dark-mode on and
fixed small stuff to make them readable. Generally I wanted to make
cards the right color and text readable. Various pages (i.e. settings,
downloads, history, bookmarks) look terrible but are actually dark now.

R=aee@chromium.org
BUG=883049

Change-Id: I8923dc7cac7680641aa5d3c4f5a35f20441dafce
Reviewed-on: https://chromium-review.googlesource.com/c/1391912
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619175}
13 files changed