Copy the video file to the /usr/share/sddm/themes/breeze-slider2/Video/VideoBackground/

Add the file name to the 'videoFile' key - example:
videoFile=VideoBackground/Rotating Earth at Night: Suomi NPP Satellite Imagery 2012 NASA.mp4

Add the icons to the 'icons' directory and add the correct paths to the button paths.
Examples:
coverSuspenButton=/usr/share/sddm/themes/breeze-slider2/icons/suspend.png
coverRebootButton=/usr/share/sddm/themes/breeze-slider2/icons/reboot.png
coverShutdownButton=/usr/share/sddm/themes/breeze-slider2/icons/shutdown.png
coverSearchButton=/usr/share/sddm/themes/breeze-slider2/icons/search.png

[General]
---Standard Breeze---
type=color
color=black
background=
---Sliding cover---
coverColor=#80000000
coverWaitTime=180000
coverClock=components/ConfigurableClock.qml
clockX=100
clockY=50
coverClockTimeFormat=HH:mm:ss
coverClockTimePointSize=32
coverClockDateFormat=dd-MM-yyyy
coverClockDatePointSize=18
coverSuspenButton=
coverRebootButton=
coverShutdownButton=
coverSearchButton=
---Extra background---
extraBackground=Video/DoublePlayer.qml
---Video background---
videoFile=
videoFillMode=PreserveAspectCrop
videoVolume=0.0
---Background Clocks----
backgroundClock=BackgroundClocks/Blinkclock.qml
backgroundClockBackgroundColor=transparent
backgroundClockBackgroundRadius=5
backgroundClockX=80
backgroundClockY=800
backgroundClockTimeFormat=HH:mm:ss
backgroundClockTimePointSize=32
backgroundClockTimeTextColor=white
backgroundClockDateFormat=dd-MM-yyyy
backgroundClockDatePointSize=18
backgroundClockDateTextColor=lightgray
