RSK theme fixes

[gabitoesmiapodo]

Related to #1672

Motivation

Main changes were implemented on #1903 but I've found a few discrepancies that I addressed here. Links to the updated design can be found at #1672

Only @import "theme/base_variables"; and @import "rsk_variables"; should be uncommented in block_scout_web/assets/css/theme/_variables.scss for this to work.

New network selector present in the design won't be styled until it's implemented.

Fixes (screenshots are before the changes):

Corrected background gradient color and orientation.
Fixed block number color.

Corrected addresses' color.
Corrected dropdown button's color.

Corrected social icons' color.

Block list items color.

Changelog

Bug Fixes

• RSK theme colors.

Checklist for your 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 if necessary

[coveralls]

Pull Request Test Coverage Report for Build 47b4d2d6-a158-4da6-b773-c5cc0c2334cf

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 80.86%

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Totals
Change from base Build 2ddffb09-f7f0-48c6-a78b-73f8832e738d:	0.0%
Covered Lines:	4816
Relevant Lines:	5956

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💛 - Coveralls