The keyword "should" seems you're asserting something.
If you simply want to make a IF, you should write something like this :
if (page.has_css?("body.bg-b")) || (page.has_css?("body.bg-c"))
# Check something with should
else
# Check something else with should
end
It you want to check the page have a css or the other one, you should write
(page.has_css?("body.bg-b")) || (page.has_css?("body.bg-c")).should == true
Or
if !page.has_css?("body.bg-b")
page.should have_css("body.bg-c")
end