Fix dark mode problems

[Tschonti]

• [ ] The hover color of the dark mode toggle button is incorrect in dark mode
• [ ] The alert at the top of the frontpage (that displays stuff from env vars) isn't readable in dark mode
• [ ] The social buttons in the footer are white in dark mode
• [ ] The rackhost logo doesn't have a background, so it's unreadable in dark mode
• [ ] the part just below the hero image on project pages is also weird in dark mode (example)