Add Dark mode to the website

[pokonski]

Description

This PR adds necessary CSS changes to allow dark mode, including sponsor logos.

• Some logos were pulled from sponsor's brand assets, some were tweaked manually to change the colors and some inverted with CSS filter property to avoid introducing new image files.
• I also made sure the text is contrasting enough to avoid accessibility issues.
• Replaced the Github star button with a newer one that supports dark mode and has the count properly showing (the old version seems broken)

Testing

Tested locally by running zola.

Screenshots

Click to view dark mode screenshots

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[pokonski]

Added hx-preserve to avoid flickering when the button is loaded during navigation

[1cg]

can you retarget against dev and we'll release this as part of htmx 2?

[pokonski]

@1cg done!

[gnat]

Well done! @pokonski