You're missing the effect of the "mere aggregation" clause (the paragraph after 5(d) in GPLv3), which means a distributor can distribute something that is "the work" alongside (and potentially intermingled with) something that isn't "the work" without causing the two to become mixed and without requiring their additional content to be GPL-licensed. The GPL only requires the two to be considered the same work if one is actually derivative of the other. It would be pretty hard to argue that CSS or images are derivative of the original code, IMHO. Javascript less so, but still tricky and could go either way. The question is do they combine to form "a larger program", or are they independent programs communicating over an open channel. The latter is a pretty convincing explanation, so they would be considered an aggregate rather than a single work by the GPL, IMO.
(This is not legal advice. Consult a qualified expert rather than rely on this.)