Improve UI design (Change some colors)

[shge]

Since the colors and gradations were a little outdated, I made some changes to the UI design. 😃

[Snuffleupagus]

Just making the entire default viewer, which is shipping in Firefox, essentially black really isn't an improvement at all. There's a number of issues such as e.g. the main toolbar being indistinguishable from the rest of the viewer, and generally a fairly incomplete attempt at changing the styling in any coherent way.

For a custom implementation you're obviously free to do whatever you want, but I don't think this is something that belongs in the main PDF.js library.

[shge]

@Snuffleupagus
Okay, I will change only selection colors.

[Snuffleupagus]

Okay, I will change only selection colors.

That's already tracked in issue #9184, and it's not really clear if/how any changes should actually be made here (since there's a degree of subjectivity involved).
As for these particular changes, they are not an improvement since for one they make the actual text less clear when it's selected (probably caused by the change in opacity).

[timvandermeij]

I agree with the above. The only thing we may consider is #9702, but the designers from Firefox should come up with a design first.