Improvements to high contrast mode support

[Dão Gottwald]

Over the last quarter, I've been focused on making Firefox properly support
high contrast mode again. The tracking bug for this is <
https://bugzilla.mozilla.org/show_bug.cgi?id=1016556>.

In case you didn't know, high contrast mode is a collection of Windows
themes tailored for users with not so good eyesight:
https://support.microsoft.com/en-us/help/13862/windows-use-high-contrast-mode

There are four high contrast themes available, three of which have black as
their base color and one black-on-white variant:
http://thewindowsclub.thewindowsclubco.netdna-cdn.com/wp-content/uploads/2015/01/high-contrast-themes-windows.jpg

High contrast mode considered an important accessibility feature. Formally
we have always supported this, but in reality our support has been far from
perfect and in many cases gotten worse over the years.

Here are a few screenshots to illustrate what kinds of bugs we've fixed:

Menu bar, search bar and about:home (before): http://imgur.com/vMXgZMm
Menu bar, search bar and about:home (after): http://imgur.com/a4Volo5
Menu panel and customize mode (before): http://imgur.com/iBXWbeA
Menu panel and customize mode (after): http://imgur.com/n881M4v

As you can see, issues range from text being completely illegible and icons
disappearing, over icons and text lacking contrast, to some parts of the UI
not adjusting at all to high contrast themes.

Support still isn't perfect but a lot better than it used to be. Going
forward we as a team will need to remind ourselves that high contrast mode
is a thing and that we're committed to supporting it. Please keep this in
mind when writing and reviewing patches. Please test your features in high
contrast mode.

You might want to take a look at the fixed bugs to get a sense of what's
needed to support high contrast mode. It often comes down to picking
platform colors instead of hardcoding color values, or carefully picking
color values that don't assume that everyone is using the same default OS
theme. If you're unsure what's needed to make your feature properly support
high contrast mode, feel free to ask me for feedback.

Regards,
Dao
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