Apologies for the delay in responding - the last few weeks have been super busy.
Never mind - we are all volunteers.
What operating system / distribution are you running? That could have an effect on the TreeView line colors. If possible, I think it would be better to use a color from the underlying GTK theme rather than hard-coding black.
I am using Ubuntu 18.04.2 and a gnome desktop. I lately implemented the option to show tree lines for the workbench plugin's sidebar and there I also see red lines. So most likely you are right - it seems to be a local problem not a general one. I will remove the CSS part which sets the border color.