Render from within partials without passing a local variable
structure=""
if(@page.theme_set = 1)                          #line 1
  render :partial => "first_theme.html", :locals => {:structure => structure}
else
  render :partial => "second_theme.html", :locals => {:structure => structure}
end
concat(structure)                                #line 2

<% structure << header %>
<% structure << content_for_first_theme %>
<% structure << footer %>

if(@page.theme_set = 1)   #this will always evaluate to true, use double equals (==) to find equality.

<%= structure << footer %>