Options → Addons tab → TipTac → Options → Backdrop → Tip Background Color & Tip Border Color → Lower the 2nd vertical slider on the right.
I have been enjoying the TipTac Reborn addon by frozn45, which relocates the Blizz UI tooltip to follow the mouse cursor. However, the default setting would also cause the tooltip to cover up Raid Frames, making it harder to heal up the teammates on the right-hand side of the frames.
So I have looked around and found that, hidden in the “TipTac → Options → Backdrop,” there is the option that allows us to make the tooltip see-through!