What icons are you missing? I don't see any icons in either the nixpkgs version nor in this PR.

qalculate (on unstable) is missing icons for me:

After adding the icon theme it looks like this:

I'm going to quote the nixpkgs manual on icon themes in gtk applications:

https://nixos.org/nixpkgs/manual/#ssec-gnome-packaging
When an application uses icons, an icon theme should be available in XDG_DATA_DIRS. The package for the default, icon-less hicolor-icon-theme contains a setup hook that will pick up icon themes from buildInputs and pass it to our wrapper. Unfortunately, relying on that would mean every user has to download the theme included in the package expression no matter their preference. For that reason, we leave the installation of icon theme on the user. If you use one of the desktop environments, you probably already have an icon theme installed.

So we prefer that we didn't add this icon theme to the package. And instead configure your system to include a icon theme for the application.

I'm going to quote the nixpkgs manual on icon themes in gtk applications:

https://nixos.org/nixpkgs/manual/#ssec-gnome-packaging
When an application uses icons, an icon theme should be available in XDG_DATA_DIRS. The package for the default, icon-less hicolor-icon-theme contains a setup hook that will pick up icon themes from buildInputs and pass it to our wrapper. Unfortunately, relying on that would mean every user has to download the theme included in the package expression no matter their preference. For that reason, we leave the installation of icon theme on the user. If you use one of the desktop environments, you probably already have an icon theme installed.

Thanks for the hint. I was not aware of this section in the manual.

After installing adwaita-icon-theme globally and logging in again, the icons are working fine now.