You will see other videos that tell you to modify the Registry to change your Cortana search bar to a dark color instead of white.  The short story is that those videos are wrong.  Hacking the registry is rarely the right thing to do and in this case, it's just plain wrong.

 

Microsoft controls the color of the Cortana Search box by the 'mode' settings of Windows. Simply by changing a single setting in the Personalization section, you can set your Theme to Dark and Cortana will change.  And unlike the Registry hack, it will be permanent.