The findbar CSS rules predate the general availability of CSS nesting, which makes them more difficult to understand and change safely. The primary issues are that the findbar rules are spread all over the viewer.css file, they share blocks with non-findbar elements and the scope of the rules is sometimes bigger than necessary.

This refactoring groups all findbar-related CSS rules together, scoped to the top-level #findbar element, for improved overview and isolation. Note that this patch only intends to move the existing rules around and not change any behavior yet, but it does lay the foundation for e.g. making the findbar respect the browser.uidensity preference in Firefox in follow-up work.

Notes for the reviewer:

• I have deliberately introduced the #findbar rules in a different part of the file to make the diff easier. Now the deletions and additions can more easily be compared side-by-side while otherwise they would be intertwined.
• The rules at https://github.com/mozilla/pdf.js/pull/18568/files#diff-d1047e1376717fbe7c554bb385cd48b2dcbbb135e02884be3db97ad072ba741eL365 were all overridden by either https://github.com/mozilla/pdf.js/pull/18568/files#diff-d1047e1376717fbe7c554bb385cd48b2dcbbb135e02884be3db97ad072ba741eL451 or the doorhanger class, and as such could simply be removed. This can be confirmed by opening the default viewer, inspecting the #findbar element and noticing that the rules in this block are already inactive:

• The other rules have been moved as-is, but only placed in alphabetical order in the new blocks for increased readability.

Extracts a part of #18385.