Send to Back
Moves the selected object to the bottom of the stacking order, so that it is behind the other objects.
Choose Format - Arrange - Send to Back (Collabora Office Writer, Collabora Office Calc).
Choose Shape - Arrange - Send to Back (Collabora Office Draw).
Shift++minus sign (Collabora Office Impress, Collabora Office Draw)
Open context menu - choose Arrange - Send to Back (Collabora Office Impress).
Send to Back