This is an XFWM4 theme based on the Classic Mac look, but modernized to support both dark mode and high DPI.
This theme is based directly on the Copland theme here: https://www.xfce-look.org/p/1204016/
Just use make and make install to install the theme. The theme
is named rain-retro-modern, with -2x up to -4x for high DPI,
and -dark for dark mode.