feat: Dark mode/display on load

[smallbrownbike]

Changes

• Posts a message to the posthog.com iframe when the theme is toggled (the theme is then toggled within the iframe as well)
• Hides iframe until a ready message is received (this should fix the nav flicker issue)
• Updates the message type logic to support the new format in this PR

[posthog-bot]

Hey @smallbrownbike! 👋
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[benjackwhite]

Made a couple of tweaks to have a better loading state and not to muddy the themeLogic but this is awesome!