Well, I've told you where to get Kvantum themes, and that the repo carries a couple of them already, but other than that, you will also find many regular Qt and GTK themes in the repos, and you will see their effects when you apply them.
As @linux-aarhus said, it is very difficult to find a combination of themes that would be consistent across Qt and GTK. The Manjaro-native Breath theme ─ which exists for both widget toolkits ─ comes close, but it's not exactly perfect, and a lot of that has to do with the (quite significant) differences, both in how Qt and GTK were designed respectively, and the more restrictive mindset of the GTK developers versus the more flexible mindset of the Qt developers.
For the themes in the repo, you can use your package manager. Just look through the different sections.