Customize overlays by selecting the text.
From the right toolbar, you can edit the text, move forward or back, change orientation with respect to the drop zone or card, and change color and opacity.
Bring the overlay forwards or backwards to overlap or layer text by selecting the icon of two overlapping squares.
Change orientation or "center" the overlay with respect to either the card or drop zone.
Select a new color and adjust opacity by clicking on the circle icon with a quarter piece blacked out.
To remove the overlay, either select the "x" icon or hit the keyboard backspace button.
Watch the Card Designer tools tutorial here.