Themes / Plugins: Ensure visual consistency between theme and plugin upload screens - #13219

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Props wprashed, sadmansakibnadvi.

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Test this pull request with WordPress Playground.

Tested this on the Playground build for the PR and posted a full test report on the ticket. Short version: at 900px both upload screens now render the help text identically, and nothing changes at 1400px. No regression on the theme screen.