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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• [x] tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with rake test
• [x] code is in uniquely-named feature branch and has no merge conflicts
• [x] PR is descriptively titled
• [x] PR body includes fixes #0000-style reference to original issue #
• [x] ask @publiclab/reviewers for help, in a comment below

[welcome[bot]]

Thanks for opening this pull request! Dangerbot will test out your code and reply in a bit with some pointers and requests. There may be some errors, but don't worry! We're here to help! 👍🎉😄

[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)