✨ Add light mode to the main landing page

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Original issue description

Is your feature request related to a problem? Please describe. So there seems to only be a light more for some parts of the landing page. It'd be great to get the whole thing with a light mode!

Describe the solution you'd like Adding global variables for our light mode colours and make sure we're explicitly calling dark:... for our dark theme.

Additional context We're using Tailwind v3 on the landing page, so it should be easy enough to extend Tailwind properly and get the light/dark mode all working correctly.

closes #49