Choose Format - Arrange - Send to Back (LibreOfficeDev Writer, LibreOfficeDev Calc).
Choose Modify - Arrange - Send to Back (LibreOfficeDev Draw).
Shift+CommandCtrl+znaménko minus (LibreOfficeDev Impress, LibreOfficeDev Draw)
Open context menu - choose Arrange - Send to Back (LibreOfficeDev Impress).