I ran into issues when I tried to use SO CSS with a set of plugins I installed.
I disabled one by one all the plugins and I found that Shield (by Icontrol WP) has security features that crash the inspector interface (#socss-inspector-interface in the source code).
I found that enabling in The Shield dashboard -> HTTP Headers -> Content Security Policy (a tab) -> and checking ‘Allow Javascript eval()‘ will resolve it.
Hope it will help some people encountering this problem, or if an update may fix that it will better.
Regards,
Louis B