TreeItem padding to separate items with outlines

[AdeptusCat]

When TreeItems are set with a custom_bg_color and just_outline=true the outlines are touching. Depending on the zoom this may look like the colors of the outlines are mixing to a new color. In the example image the colors green and red mix to yellow.
The margin theme option is not helping, since a padding would be necessary. This is a suggestion to implement padding for TreeItems.