Themes: When the user doesn't have switch_themes capabilities, and we've passed the current theme, detect the theme slug/stylesheet correctly so that the flags are set properly. See #25948

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Themes: Do extra Cap checks before outputting links, and use full URI's. See #25948

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Remove RTL and MP6 references from colors-fresh.css as they're now irrelevant to the file.

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Themes: Restore a label accidentally removed in r26470. Props SergeyBiryukov. See #25948

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