Instead of outputting your processed SASS files to /_site/css , set only /css (the root level of the site) for output.
What happens is that locally SASS is working fine and output to your /_site/ directory, as you expected. But when creating on GitHub pages (where jekyll-compass gem is not supported), your CSS is not displayed on /_site at all, because the plugin does not execute there.
It doesnβt matter that it works locally from there, because GitHub Pages runs the jekyll build command jekyll build after clicking and creating /_site/ again. So everything that it does not support (i.e. Jekyll-compass) does not fall into the production version of your /_site folder.
The workaround I suggested works because, instead of outputting the final CSS to a directory that is rewritten when you click on the GitHub pages, it instead writes it to a directory that is saved even with the new build /_site/ .
Also note that Jekyll 2.0 will support SASS, and you can even use 2.0 alpha gem locally if you want (although GH Pages won 'before upgrading to 2.0 until it is officially released).
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