Improve dark theme

[zebreus]

This PR implements changes to the dark theme (#730) based on user feedback. A preview deployment is available at search.wirs.ing.

Feedback

I received the following feedback:

• The header bar should have a different color than the main content
• White text for everything has too much contrast
• The sort dropdown should have a different background color
• The results do not look like links because they are the same color as the text
• Bad contrast of hovered links

This PR addresses all of these points.

Remarks

While the result is still not optimal, as now all text is grey, there is no easy fix for this. A better solution would be to redesign search.nixos.org to align its styling more closely with nixos.org, but that would require a lot more work.

[SuperSandro2000]

Now it feels a lot better already. Thanks!