Add fallback for dark theme when localStorage isn't available

[mayankkamboj47]

Make the dark theme work on browsers that have disabled cookies and localStorage access

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Thanks for opening this pull request! Dangerbot will test out your code and reply in a bit with some pointers and requests.
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[mayankkamboj47]

@publiclab/reviewers I do not have anything to ask except that, perhaps the last point shouldn't be a list-item to check ?

(Okay, now I can check the last item)