Extended Theme for Background and Title

I'm facing the same issue too. I attended a Microsoft hosted Power BI Community call last week when they showed the enhanced theming. I asked whether we could control backgrounds using the enhanced themes. The answer I received was "visualisation backgrounds yes, page backgrounds no".

Like you I want to set the visualisation backgrounds using a theme, but I can't get it to work. I'll keep trying and if I find the solution I'll let you know. Regards, Mark

Sorry for the non-helpful response, but I was just about to post this same thing!  I want a standard look for my page backgrounds and visuals' borders, but I cannot find a way to do it.  Either the documentation (posted by David Iseminger here) is incomplete, or it fails to warn that there are features missing from the preview release.

The documentation lists the visual names and card names, but I cannot find a list of valid property names and values for those cards.  All I have at my disposal is the partial list in Mr. Iseminger's post, and the property names and values given in the example themes in the themes gallery (e.g. "fontSize":8, "show":true, "responsive":true  -- these are in the examples but not the documentation). 

Using the provided documentation, I cannot even figure out how to change the default font color of something.  Is it "fontColor"? "textColor"?  I'm just guessing (usually wrongly).

Is there a reference guide for ALL card propertyName: propertyValue possibilities somewhere?  Are they part of a larger API library or something?  If so, the documentation really needs to point this out.