Afaik the "original" Lightly was born as a fork of Breeze, which in turn was born a fork of KDE4's Oxygen. So all of them are written in pure C++.
Now, I heard Luwx/Lightly was stalled so they forked it in boehs/Lightly, merged some pending patches and even did a new branch to port it to Qt6 - but last time I tried to compile it, it failed. Not sure if they're still working on it, though. (From my part never liked Breeze but found about Brise, which I found much more torerable).
If I were you I'd try to get in touch with the mantainers of boehs/Lightly. If that doesn't work, I'd go to ask the KDE VDG (I guess they should be reachable at discuss.kde.org); at least they should redirect you to someone versed in Qt C++ styling - which is very complicated, at least for me, 'cause C++ is no easy thing and it seems there's almost no documentation at all about the subject. Pinheiro himself struggled to find someone with enough knowledge of C++ to help him with his O^2 theme.
If that doesn't work either for whatever reason, which I doubt ever happens, I'd try asking Carl Schwan as a last resort, the guy that came up with Brise. He helped me with a stupid patch for it - of course he knows his thing and seems to be very cool.