refactor: viewer.css refactored, duplications removed

[Zdobnov]

Context

This ticket requires refactoring of the viewer.css file: #11264

What was done

• border- & box-shadow duplications removed;

[Snuffleupagus]

refactored all z-index values, as there is no necessity to have such a huge abyss between 10k & 30k (nobody will use 13,876 for example).

That doesn't seem like it belong in this PR, since all of the other changes are essentially "mechanical" and probably won't require that much testing.
There's also the question if we even want to change this, and if so how, since I believe that the z-indexs were purposely implemented such that there's some "space" between them to more easily avoid errors.

[Zdobnov]

@Snuffleupagus if it's necessary to implement something new with z-index, between 5 and 10 values there are still 4 steps. In the previous solution values where 199 and 200, and 10k, 20k, etc. at the same moment. It was a bit confused.
Anyway, if that's a problem, I can remove all changes excluding duplications avoiding, just let me know.

[Snuffleupagus]

Anyway, if that's a problem, I can remove all changes excluding duplications avoiding, just let me know.

As is hopefully clear from #11268 (comment), I obviously think that part should be reverted.
(Also, it's always best if the commit message itself contains all of the information from the PR description.)

[Zdobnov]

@Snuffleupagus thank you for advice. I did some changes, now the PR contains only necessary changes, may I ask you to check it?
Thank you.

[Snuffleupagus]

/botio-linux preview

[pdfjsbot]

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[pdfjsbot]

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Published

• Viewer: http://54.67.70.0:8877/53aa6f1c4af06c5/web/viewer.html

[timvandermeij]

Thank you for your contribution!