If you into Skinstudio you can change that part with a dark color graphics. Here is how:-
Open Skinstudio. Load Corporate.
1) Go to Edit Titlebars and frames.
2) Select UIS2 Frames : Select Non captioned window edge.
3) On Image tab (lower left) Need to change that image to something else.
4) Click on dropdown arrow.
5) Search for and select "min_Window_caption.bmp" look like this part compatible for "Non caption Windows edge"
6) Save the skin.
7) Appy skin.
8) Result if it was done correctly, you should now have that on your system.
Stardock Community Assistant.