try this stuff dark themed

config/xsession/xsession.nix

@@ -9,6 +9,15 @@
 
   xsession = {
     enable = true;
+
+
+    initExtra = ''
+export QT_QPA_PLATFORMTHEME="qt5ct"
+export QT_PLUGIN_PATH=$HOME/.kde4/lib/kde4/plugins/:/usr/lib/kde4/plugins/
+XDG_CURRENT_DESKTOP=KDE
+export DESKTOP_SESSION=kde
+    '';
+
     windowManager = {
       i3.enable = true;
       i3.config = {
@@ -51,6 +60,24 @@
     backend = "xrender";
   };
 
+
+  gtk = {
+    enable = true;
+    theme = {
+      package = pkgs.breeze-gtk;
+      name = "Breeze-Dark";
+    };
+    iconTheme = {
+      package = pkgs.breeze-icons;
+      name = "Breeze Dark";
+    };
+  };
+
+  qt = {
+    enable = true;
+    useGtkTheme = true;
+  };
+
   home.packages = [
     pkgs.xorg.xbacklight
     pkgs.pavucontrol

home.nix

@@ -22,6 +22,7 @@
     
     pkgs.firefox
     pkgs.mpv
+    pkgs.sxiv
     pkgs.spotify
     
     pkgs.mumble