Dark theme import to the end of sass

[vbaranov]

Motivation

Changelog

Bug Fixes

Dark theme import should be at the end of sass file in order to apply it to all styles. Unintentionally changed before in this commit 77b7d34#diff-5cbc16d38bf2aa42b4b2ac22b572ceb7R78

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
• If I added/changed/removed ENV var, I should update the list of env vars in https://github.com/poanetwork/blockscout/blob/master/docs/env-variables.md to reflect changes in the table here https://poanetwork.github.io/blockscout/#/env-variables?id=blockscout-env-variables. I've set master in the Version column.
• If I add new indices into DB, I checked, that they don't redundant with PGHero or other tools