This video from nicco explains it. Tldw KDE uses 3 or 4 different frameworks to decorate its apps so making a theme is very complex. Union is a bridge between the user and these frameworks so the user can make a theme using whatever formats they want and union will convert it into a format the different KDE frameworks can understand.
https://youtu.be/920faq0S5sE
This video from nicco explains it. Tldw KDE uses 3 or 4 different frameworks to decorate its apps so making a theme is very complex. Union is a bridge between the user and these frameworks so the user can make a theme using whatever formats they want and union will convert it into a format the different KDE frameworks can understand.