This review request covers several commits, starting to clean up and

fix the Review Board color scheme. We have a lot of hard-coded colors

throughout the stylesheets, many of which are unintentional variations

on the same set of basic colors.
To start, this adds new definitions for the different shades of grey,

blue, brown, green, red, and yellow. We also had a number of defined

colors which weren't used very much (or at all), which have now been

properly commented as deprecated.
The actual changes to the styles cover:

The background color assigned to input elements in the upload-diff

  mixin evaluated to white, so I've just removed the rule.
The border color for the drag-hover in the upload-diff mixin was so

  close to the normal border color as to be indistinguishable.
The issue summary table had a yellowish border on mobile only, which

  looked kind of weird (the tabs still used a grey border). I've removed

  the special rule and now everything uses the same grey border.
Uses of @grey and @light-blue have been changed to use the new

  color definitions.
Started converting the main page colors over to the new color

  definitions, primarily in the .page-colors mixin, which controls the

  color scheme of the header bar.
Changed the definition of the link color to blue-20, which is a dark

  teal-like color (though there are a lot of links throughout the UI

  which do not properly use the link-color definition yet).

While I was in here, I noticed that the scrollable portion of the "New

Review Request" page would overflow the rounded corners on Chrome.

There's a weird hack that adding a null transform fixes this, which I've

added in.

Verified the different page styles (base, reviewable, admin) and

checked that the affected styles looked correct.