In AppStyling go to the Tabs tab in the Canvas. There is a Canvas Options dropdown over the canvas tabs. Go to close buttons and set the Close Button Location property. Hover over close button and hit "1" for The close button ui role. On the left in the UI Role tree, go up the chain until you see the TabCloseButton UI element. Set the Image property for the Normal, HotTracked, and Pressed settings for that UI Role.
I set the image property for the background and did not see a black x. I attached my isl.
That was it. I was able to see what you were mentioning. To get rid of the X, set the image.alpha to transparent.