ENH: Minor updates and tweaks to Dark theme CSS

[12rambau]

I solved som small issues that I saw in the dark theme by using it:

• change the color of the search field input (it was displayed in grey so completely invisible)
• use variable for sidebar border color
• change target coloring (the used light blue was not working with the new H1 colors and the spans were still in yellow)

PS:
Fix #630

[choldgraf]

This looks good to me in general - though why did you decide to revert the search bar design updates? I believe those were added in #578

[12rambau]

Because I saw it in the "stable" version of the doc. I thought that was a mistake I made in #540.
@choldgraf lucky me that was the last commit. I've reverted it so now I'm not changing the search bar design any more

[choldgraf]

This LGTM - thanks for the improvements and clean-up after the first PR!