Add dark mode support

[VeckoTheGecko]

This PR adds website support for Dark mode via a toggle in the top right.

Set as draft for visibility and to see if this is something we want to go forward with. Still need to fix contrast on the fonts and the images. For images that would require providing a dark mode compatible image (using the new ColorModeImage component) or otherwise changing the representation within the website.

Fixes #115

[erikvansebille]

Thanks for exploring! How would the ColorModeImage() then work in practice? There is already a dianegative UU logo in /public/funders-logos/UUlogo_dia.png; how would we code that that one gets chosen?

[VeckoTheGecko]

How would the ColorModeImage() then work in practice?

import { ColorModeImage } from './components/ColorModeImage'

...
    <ColorModeImage
      lightSrc="/funders-logos/UUlogo.png" // image to show in light mode
      darkSrc="/funders-logos/UUlogo_dia.png" // image to show in dark mode
      alt="Company Logo"
      width="200px" // any other attributes you'd normally put on <Image/>
      height="auto"
    />
...

Then the image being shown would change accordingly

[erikvansebille]

I think I'm done updating all the images to a dark-version too. The homepage animation could be slightly cleaner (without the small white border around it), but I'll do that later. What do you think? Shall we merge this, @VeckoTheGecko?

[VeckoTheGecko]

I think this looks great! Only comments are that we have some low contrast of the "days since" text (now fixed in 67d2bb5), and also we should be mindful that in blog posts we won't be as flexible with providing assets for both light and dark mode.

Happy to go ahead, feel free to merge :)