Eliminate dark background except Dark forest theme

[vbaranov]

Motivation

Dark background applied for opened modal window.

Changelog

Apply dark background for modals only for Dark forest game theme

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 dcc545566f926c77cc69dd961459b077c5965cd1-PR-3371

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 67.459%

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Totals
Change from base Build f2d503ba4c33e4868ed10ff6f6cb3c3b3a456b4d:	0.7%
Covered Lines:	2044
Relevant Lines:	3030

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💛 - Coveralls