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macOS Sierra 10.12.6: for me the Mac App Store updates lost their styling, too :-(
MacAppStoreWithoutStyling.png

This reminds me somewhat of Mac OS X Mountain Lion 10.8. where a newer unsupported TLS version prevented to check mail on certain services and refused to load websites on Safari. Probably it's a security feature, not a bug :)
 
Yes, you're probably right, but maybe Apple moved this specific CSS to some server that older OSs can't reach for whatever reason.
 
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The strange thing is, that Mac OS X Mavericks 10.9.5 Mac App Store updates look perfectly fine.
 
Unbelievable, but really cool that Apple fixed it. That makes macOS 10.12. look great again.:)
 
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