Themes: Move the escaping of content from JS back to PHP. This allows us to take advantage of the display() WP_Theme method to translate the text properly, and to strip out any HTML tags we don't wish to display. Fixes #26100. See #25948

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Theme Installer: Fix the Theme Preview function, and bring the button handling/styling inline with the Customizer. Fixes #26129

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Restore "Skip to toolbar" link color. fixes #26146.
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