[Rails] Rails reCaptcha gem - Not accepting theme option identified in Google's docs

I'm using the recaptcha gem (github.com/ambethia/recaptcha) but have been
unable to change from the default theme. The google documentation indicates
that there are four base themes, one of which is 'white'. The recaptcha_tags docs
identify a :display option:

:display - Takes a hash containing the theme and tabindex options per the API. (default nil)

I'm calling the function this way:

<%= recaptcha_tags :display => { :theme => "white" } %>

But I only get the default theme, not the white theme. I inserted a puts()
into the source of the recaptcha gem where the options are parsed and get
the following:

Options: {:public_key=>"XXXXXXXXXXXXX-YYYYYYYYYYYYYYY", :error=>"expression"}

I've tried every variation I can think of to pass a :display option with a
hash of :theme => "white" and can't get it to work. What am I doing wrong
here?