You can get the PKGBUILD in Manjaro's Gitlab.
Get the PKGBUILD, make the package with
makepkg -s, and install it with
pacman -U /path/to/package (or you can use
makepkg -si to build the package and install it right after, without calling pacman manually after building the package).
(At first glance, there seems to be no make dependency that is Manjaro-exclusive. You might want to do some cleanup in the optional dependencies though, like removing
'kvantum-manjaro: qt5 engine for adapta'.)
Since it's a GTK theme, installing the prebuilt package in Manjaro might work fine in Arch, but I can't guarantee it. You can get any packages that you want in mirrors, for example https://osmirror.org/manjaro/stable/. The Adapta-Maia theme is in Community.