User blog comment:BertH/Fluid layout is arriving on December 4/@comment-4519283-20131118204633

I don't think people know what this is. "Fluid Layout" simply means that the browser shrinks with the content. Try resizing this window (left-to-right), which has Fluid Layout enabled, compared with another wiki that does not have Fluid Layout enabled.

The Special:CSS page is just a simpler way to manage CSS on your wiki, if you've never used CSS it won't affect you in any way.

The theme designer updates simply give you access to a few new options.