Fix dark theme flickering

[vbaranov]

Motivation

When the page is loading or user switches between pages with the dark mode applied, the light mode is loading first and then dark mode applied after a while.

Changelog

Bug Fixes

Raise the priority of dark theme class applying to window.onload event.

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

[ayrat555]

@vbaranov gettext is failing

[vbaranov]

@vbaranov gettext is failing

Thanks, fixed