Convert the various colors in web/viewer.css to CSS variables

[timvandermeij]

To make re-skinning the viewer easier, the various repeated colors in web/viewer.css should be converted to CSS variables. This way they should be easier to tweak for third-party deployments of PDF.js.

[Snuffleupagus]

Before we start converting everything but the kitchen sink to use CSS variables, we may want to avoid adding "too many" until the performance impact (in Firefox) of doing so is better understood; based on stumbling upon https://groups.google.com/d/msg/firefox-dev/wQKKyH4vXks/iFmRdHBcEwAJ and https://bugzilla.mozilla.org/show_bug.cgi?id=1561001#c4 yesterday.

Edit: I obviously don't know exactly how many is too many, but we probably want to avoid adding hundreds of CSS variables.
Since there's already issues tracking the modernization/clean-up of the viewer UI, it'd probably make the most sense to introduce CSS variables as part of these improvements (which really need to happen iteratively, since changing the entire UI in one big commit is essentially impossible to test/review).

[rohanharikr]

Would like to work on this!

[Snuffleupagus]

Would like to work on this!

As already mentioned in #11572 (comment), this would be best done as part of other CSS modernization/improvement work:

Since there's already issues tracking the modernization/clean-up of the viewer UI, it'd probably make the most sense to introduce CSS variables as part of these improvements (which really need to happen iteratively, since changing the entire UI in one big commit is essentially impossible to test/review).

[rohanharikr]

@Snuffleupagus Got it!

[mitchell-frost]

I am new to mozilla. Can I work on this?

[timvandermeij]

Fixed by #11077.