To make TabControl or TabStrip control background (the area behind tabs) transparent you need to set following properties to Color.Transparent value:

C#

TabControl.ColorScheme.TabBackground = Color.Transparent;
TabControl.ColorScheme.TabBackground2 = Color.Transparent;
TabControl.BackColor = Color.Transparent;

VB

TabControl.ColorScheme.TabBackground = Color.Transparent
TabControl.ColorScheme.TabBackground2 = Color.Transparent
TabControl.BackColor = Color.Transparent