Reinstalling opera developer solved the problem

Since Opera developer for Mac, version 40.0.2296.0, the backgrounds are not displayed properly

Screenshot

This is how Opera reports javascript errors (in the console):
"Uncaught TypeError: undefined is not a function"

And this is how Safari does the same thing:
"Uncaught TypeError: undefined is not a function (evaluating 'foo.bar()')"

This little extra piece of information is saving me a great deal of time when developing large scale javascript applications. Please add it to Opera.