Add custom theme support with wallpaper-based color extraction (#6)

* Add custom theme support with wallpaper-based color extraction

- Extended theme option to accept "custom" value
- Added customTheme configuration with wallpaperPath and variant options
- Integrated nix-colors colorSchemeFromPicture for dynamic color generation
- Updated wallpaper selection to handle custom paths
- Added VSCode theme fallback for custom color schemes
- Fixed nix-colors contrib function call (requires pkgs argument)
- Maintains full backward compatibility with existing themes

Users can now generate themes from their own wallpapers:
  omarchy = {
    theme = "custom";
    customTheme = {
      wallpaperPath = ./wallpapers/my-image.png;
      variant = "dark"; # or "light"
    };
  };

* Fix: Screen sharing broken due to Qt library version mismatch

  File changed: modules/nixos/hyprland.nix

  Problem:
  The omarchy-nix configuration was using xdg-desktop-portal-hyprland from the hyprland flake input, which had a Qt library version mismatch (6.9.1 vs 6.9.0). This caused the
   hyprland-share-picker to crash with "Cannot mix incompatible Qt library" errors, preventing screen sharing from working.

  Solution:
  Changed the portalPackage from the hyprland flake version to the stable nixpkgs version:

  - portalPackage = inputs.hyprland.packages.${pkgs.stdenv.hostPlatform.system}.xdg-desktop-portal-hyprland;
  + portalPackage = pkgs.xdg-desktop-portal-hyprland; # Use stable nixpkgs version to fix Qt version mismatch

  Why this works:
  - The stable nixpkgs version of xdg-desktop-portal-hyprland has consistent Qt library versions
  - This matches the working configuration used on other NixOS Hyprland setups
  - Maintains all screen sharing functionality while avoiding the Qt version conflict

README.md

@@ -56,7 +56,34 @@ Once ready, add this flake to your system configuration, you'll also need [home-
 
 I've specified some basic configuration options to help you get started with initial setup, as well as some simple overrides for common configuration settings I found I was modifying often. These are likely subject to change with future versions as I iron things out.
 
-Refer to [the root configuration](https://github.com/henrysipp/omarchy-nix/blob/main/config.nix) file for more information on what options are available. 
+Refer to [the root configuration](https://github.com/henrysipp/omarchy-nix/blob/main/config.nix) file for more information on what options are available.
+
+### Themes
+
+Omarchy-nix includes several predefined themes:
+- `tokyo-night` (default)
+- `kanagawa`
+- `everforest`
+- `catppuccin`
+- `nord`
+- `gruvbox`
+- `gruvbox-light`
+
+You can also generate a custom theme from any wallpaper image:
+
+```nix
+{
+  omarchy = {
+    theme = "custom";
+    customTheme = {
+      wallpaperPath = ./path/to/your/wallpaper.png;
+      variant = "dark"; # or "light" for light themes
+    };
+  };
+}
+```
+
+This will automatically extract colors from your wallpaper and generate a matching color scheme for all Omarchy applications (terminal, editor, launcher, etc.). 
 
 ## License