You can Cortana's search box highlight transparency with a simple Registry Tweak. This change has been spotted in Windows 10 build 15014. Let's see how it can be done.
To change Cortana Search Box Text Transparency in Windows 10, you need to apply a Registry tweak. Open Registry Editor and go to the following Registry key:
Repeat the same under
Restart Explorer and you are done! The text in the search box of Cortana will be semi-transparent.
Here is how Cortana looks out of the box.
Here is the example of the opaque highlight:
This is how the transparent highlight looks like:
In this mode, it is a good idea to enable White Text in Cortana (Search Box). See this screenshot: