The icon theme that Zrythm uses is freedesktop-compliant, so it can be easily overridden.
To override the Zrythm icon theme with your own icon theme,
you need to create a directory similar to
under your Zrythm installation directory.
if your Zrythm directory is
/home/me/zrythm, you should
create a directory at
/home/me/zrythm/themes/icons/zrythm-dark with an
index.theme file and the icons you wish to override.
An easy way to do it is by copying the directory that Zrythm comes with and replacing the icons with your own.
A more elegant way to do it is by following the Freedesktop Icon Theme Specification.