Yes, the same code located in the UI.zip file is what’s being hosted on ui.opensprinkler.com.
The error you are getting is because a special file type is being used for the CSS which is CGZ (this is just a CSS file that’s been pre-gzipped). We allowed the Arduino version to support its own web files which needed to be as small as possible (plus reading headers and gzipping on the fly wasn’t really possible). This is why we are using this format. In order for your browser to understand this is gzipped, we have to tell it. This is where the steps in the documentation become important: https://openthings.freshdesk.com/support/solutions/articles/5000164006-using-a-different-server-for-ui-assets. Particularly under Option 2, Step 1, Part 3.