That highlight color is controlled at the OS level.
There might be somewhere in a program file to change the background color of the text field, to off-set the highlight color given by the OS, but I wouldn't know where to start looking.
To change the highlight in Windows7, you need to go into Personalize > Window Color > Advanced Appearance Settings > Item > Selected Items and choose a new color.
I'm not sure where it would be in Mac, or other versions of Windows though.
As I change themes, I guess the highlight color does stay the same, it is just the "Light" theme is too close in color. I may have to change themes. I am using a MAC, OS Sierra
With that said, adjusting my MAC did help a little, but I really was happy with the default settings for everything else I did on the MAC, so I changed the MAC back to the default and just used a light setting in the DW preferences.
However, I really would like to customize a theme to better fit how I want to see my code. When I go to "edit" on my new custom theme it is total Greek to me. I don't understand anything in that file, as in where to find what I want to change. Is there a cheat sheet or instructions on how to make these changes. For instance, changing the comment colors? Maybe making the over-all font size a little bigger?
Unfortunately, there isn't much out there yet in the way of documentation from Adobe.
DW only recently changed to the partial implementation of the Brackets code engine. So some items that do work in Brackets (which you can find better documentation for online) don't necessarily function in DW.
Here's Adobe's official page: Learn how you can customize the colors of your code elements in Dreamweaver.
It does require some coding knowledge, unlike CC2015 and earlier where everything was built into the Preferences interface.
I went to the link in the main.less file and it is way over my head, however I tried to change the code for the comments and it did nothing. I think I may just have to leave it alone until there is better documentation for us unfamiliar with this.
I realized I forgot to go back and reselect the new them in preferences after the edit. It worked.
I found a little trick to find what is what in the main.less code that is provided. I had no idea what each line meant and where to look for what I wanted to change.
I took a small screen shot of the color that is currently being shown in code view. Then go to Photoshop or other color editor and select the color in that program to find out what the actual hex code color is. (#______). Go back to your main.less file in DW and do a "find" for that color. There it is! I changed a few colors this way.