In src/main/resources/web/atlantic/zul/less/norm.less
the following line is setting http:// so you get errors when using this theme under https
@import ~"url(http://fonts.googleapis.com/css?family=Open+Sans:400,300,600,700)";

Suggested fix – change the line to:
@import ~"url(//fonts.googleapis.com/css?family=Open+Sans:400,300,600,700)";

I've done this on my local build to solve the problem but it would be nice if it was upstream.

you may also want to look at line 249 of the same file to remove the http there also.
background-image: url('http://www.zkoss.org/zk/img/${z:encodeWithZK("zkpowered.png")}');