stylix: Dynamically generate wallpaper from color scheme

Fixes an issue where previously the background color would be incorrect
when the color scheme was changed, which was particularly noticeable on
Phosh where the background would display when toggling the on-screen
keyboard.

Note that the logo has been intentionally left out this time to simplify
things a bit.

modules/desktop.nix

@@ -4,11 +4,28 @@ let
   inherit (lib) mkEnableOption mkIf mkMerge mkOption;
   inherit (lib.types) float int;
   inherit (config.modules.system) username;
+  inherit (config.lib.stylix.colors) base00;
   inherit (cfg) bloat gnome plasma container opacity fontSize graphical;
   inherit (nix-config.packages.${pkgs.system}) aleo-fonts;
-  inherit (pkgs) phinger-cursors noto-fonts-cjk-sans maple-mono noto-fonts-emoji;
+  inherit (pkgs) phinger-cursors noto-fonts-cjk-sans maple-mono noto-fonts-emoji stdenvNoCC imagemagick;
   inherit (builtins) attrValues;
 
+  stylix-background = stdenvNoCC.mkDerivation {
+    pname = "stylix-background";
+    version = base00;
+
+    dontUnpack = true;
+
+    nativeBuildInputs = [
+      imagemagick
+    ];
+
+    postInstall = ''
+      mkdir -p $out
+      magick -size 1x1 xc:#${base00} $out/wallpaper.png
+    '';
+  };
+
   cfg = config.modules.desktop;
 in
 {
@@ -170,7 +187,7 @@ in
 
     stylix = {
       enable = true;
-      image = ../assets/wallpaper.png;
+      image = "${stylix-background}/wallpaper.png";
       polarity = "dark";
       base16Scheme = ../assets/selenized-black.yaml;