Search highlighting in some themes could be improved

[vsccarl]

• VSCode Version: 0.10.12-alpha (latest)
• OS Version: Windows 10

Steps to Reproduce:

1. Search for something that occurs in a file many times
2. Click between the results in the same file.

Actual:
Very low visibility that your selection has changed between the results in the same file.

Also with the Solarized Light theme some text is almost unreadable when selected by search

[nathantschultz]

It would be nice if the user could customize the search results highlight color. It's really hard to tell what is highlighted and what isn't when searching.

[sandy081]

This is fixed by #11095