Redesign dark mode

[querkmachine]

Redesign dark mode to use a more purple/green colour scheme, inverting the light mode colours instead of using an entirely new palette.

Code things to change

• [x] Update dark mode color scheme
• [x] Update theme-color meta tag
• [x] Update favicon colour scheme to match new dark mode

Content to change

• [x] Update brand page
• [ ] Update website button(?)