[skim237]

I had the same problem on my 5K iMac. Found a fix for everyone using a high res display by opening Pro Tools in "Low Resolution Mode" from Apple's support page:

"Opening apps in Low Resolution mode:
If you notice that an app looks different than you expect on your Retina display, try opening the app using Low Resolution mode:
1. Quit the app if it's open.
2. In the Finder, choose Applications from the Go menu.
3. In the Applications folder that opens, click the app's icon so it's highlighted.
4. Choose Get Info from the File menu.
5. Place a checkmark next to "Open in Low Resolution"
6. Close the window and open the app again."

This made the lines dark and editing to the grid much easier, especially in warp mode.