Let’s have a little deep dive on why this icon theme may have broken things. More or less it was this change: Inherit from Adwaita (!3) · Merge requests · GNOME / Adwaita Legacy · GitLab
adwaita-icon-theme-legacy is a dependency adwaita-icon-theme which is a dependency of gtk3 and gtk4 - most likely is also installed on your system. Adding Inherits=Adwaita to a theme you may not use might create issues as in some implementations don’t support circular inheritance. This happened with steam, electron or the flatpak apps. They looked for the Adwaita theme, which might not been active on your system. Before it only looked for hicolor. So they kept crashing cos the legacy theme depended on adwaita which itself depends on the legacy theme. So a non-working cycle was created.
Here an example for python code:
class A(B):
#body of class A
class B(A):
#body of class B
This code won’t work because B is not defined when class A got created. It proves the concept of circular inheritance. class A inherits from B and B from A… so you’re just creating a loop of inheritance, and python would not be able to evaluate this since there’s no way to look at any class individually.