Hi danperk, so actually there is a theme setting that you can use to modify the drag rectangle color.

There isn't a control for it in the theme editor editor UI but you can modify it by changing the theme colors directly.

To do that in the theme dialog (Options > General > "Manage themes") click the "Open themes folder" button and inside your theme's folder edit the defines.inc file in a text editor.

Inside the defines.inc file, the setting you need to modify is this one:
LINE_UNSELECTED_COLOR #000000

- Michael