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For SonicDE, to get rid of the lock screen default, change the name of the Next directory to Next.bak and whatever directory/image you want, make a copy of that one called Next:
mv /usr/share/wallpapers/Next /usr/share/wallpapers/Next.bak cp -ar /usr/share/wallpapers/Arc-Dark /usr/share/wallpapers/Next
As for the lock screen default image (Silver sddm mgr), change the following:
mv /usr/share/sddm/themes/Sonic-Silver/assets/bg.png /usr/share/sddm/themes/Sonic-Silver/assets/bg.png.bak cp -ar /usr/share/wallpapers/Arc-Dark/contents/images/2560x1440.png /usr/share/sddm/themes/Sonic-Silver/assets/bg.png mv /usr/share/sddm/themes/Sonic-Silver-Light/assets/bg.png /usr/share/sddm/themes/Sonic-Silver-Light/assets/bg.png.bak cp -ar /usr/share/wallpapers/Arc-Dark/contents/images/2560x1440.png /usr/share/sddm/themes/Sonic-Silver-Light/assets/bg.png
Go to software store and install network manager tray.