When I watched a youtube video of KDE customization, it types a command “pamac build lightly-qt”. What would be the equivalent command in Opensuse?
The youtube link is https://www.youtube.com/watch?v=exQh0_JKBJQ&list=WL&index=11&t=162s
Could anyone help me?
There is none. All packages that you install in openSUSE are precompiled binaries, you do not build them on end system. There is no infrastructure to deliver sources, build and install resulting binary in one go.
Lightly theme should be available for Leap 15.3 already, just go to YaST Software Manager and search for it, or do “zypper install lightly”.