As best I can tell this isn’t a problem with Fluency as the URLs that are used for the stylesheet are derived from the WordPress globals.
eg. I use WP_PLUGIN_URL, which itself uses WP_CONTENT_URL which uses the value that you set for “site url” in your WordPress settings.
Only thing I can think of that might cause an issue is if *only* your wp-admin uses https, and the front-end of your site doesn’t – if that’s the case I’m curious as to why the other admin resources don’t have this issue as well…. unless the default WP admins styles are included using a different method to what I’m presuming… hmm… will have to investigate further.
UPDATE: Ok, did a bit of digging and think I found a different way for me to retrieve the required URL that should maintain the http/https as needed.