Interestingly, at least with Firefox on Linux it seems to be decompressing the .css files ok... but borking on the javascript file that has the same incorrect content encoding header.
And the size difference appears to be only 0.2kB? 6.1kB from the zipped version and 6.3kB for the unzipped one
I see advanced.css uncompressed size is 25K and compressed (by zopfli) - 5.4K