Read panel theme setting

I’ve added a custom theme that changed the coloring to my liking. The theme is darker and I’ve set the Read Panel background to my desired color. BUT there is still a WHITE band around the left and top edges of the read panel. Is it possible to change the color of the white strip to match the read panel background? Perhaps one of the Theme variables? The attached image shows the white strip I’m referring to. Thanks a bunch in advance for any assistance.

Hi Gene,
I can only suggest playing around with the theme variables in the text editor, I am not sure which one is for the message detail border.
Maybe some of the other users who made their own themes previously might know.