Kiwix frontend style cleanup

[veloman-yunkan]

Addresses #917

Unfortunately, I failed to achieve consistent look-and-feel across the library page and the viewer toolbar so that they both look nice.

• large border radius of the library page doesn't look good when applied to the small buttons of the viewer toolbar, whereas small radius used for the viewer buttons looks inferior to what we have now on the library page
• color schemes of the buttons and filters could not be unified too; viewer current button icons don't look nice on dark background, whereas the current design of the toolbar of the library page seems to rely on contrast.

This PR simply reduces the gap in the look-and-feel of the two toolbars. If we want to eliminate it completely, I think that we must come up with a new design first.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 38.92%. Comparing base (a5db4a1) to head (29c33a7). Report is 219 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1009 +/- ## ======================================= Coverage 38.92% 38.92% ======================================= Files 58 58 Lines 3990 3990 Branches 2201 2201 ======================================= Hits 1553 1553 Misses 1090 1090 Partials 1347 1347 ```

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[kelson42]

@veloman-yunkan Thank you for this PR and this attempt to fix this annoying issue. I know this is not easy. Will give it a test this WE.