Dark theme improvements

[vbaranov]

Motivation

Changelog

Upgrading

If you have any Incompatible Changes in the above Changelog, outline how users of prior versions can upgrade once this PR lands or when reviewers are testing locally. A common upgrading step is "Database reset and re-index required".

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 347e68f1200ad819e3e7fff7bd8e96d00e4ed4f4-PR-3325

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+9.7%) to 74.699%

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Totals
Change from base Build 5821f3af3713049b3d6827c11a47106b2a243e74:	9.7%
Covered Lines:	1984
Relevant Lines:	2656

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💛 - Coveralls