lumina.lumina: update icon theme caches

pkgs/desktops/lumina/lumina/default.nix

@@ -2,6 +2,7 @@
 , mkDerivation
 , fetchFromGitHub
 , fluxbox
+, hicolor-icon-theme
 , libarchive
 , numlockx
 , qmake
@@ -34,6 +35,7 @@ mkDerivation rec {
 
   buildInputs = [
     fluxbox # window manager for Lumina DE
+    hicolor-icon-theme
     libarchive # make `bsdtar` available for lumina-archiver
     numlockx # required for changing state of numlock at login
     qtbase
@@ -48,6 +50,8 @@ mkDerivation rec {
     xscreensaver
   ];
 
+  dontDropIconThemeCache = true;
+
   patches = [
     ./LuminaOS-NixOS.cpp.patch
   ];
@@ -85,6 +89,12 @@ mkDerivation rec {
     done
   '';
 
+  postInstall = ''
+    for theme in lumina-icons material-design-{dark,light}; do
+      gtk-update-icon-cache $out/share/icons/$theme
+    done
+  '';
+
   qmakeFlags = [
     "LINUX_DISTRO=NixOS"
     "CONFIG+=WITH_I18N"