fish: support theme plugins

Allows for linking fish plugin theme files to [fish's themes
folder](https://fishshell.com/docs/current/cmds/fish_config.html#theme-files).
Fully allowing for theme plugins to properly be installed declaratively.
Fixes nix-community/home-manager#3724

modules/programs/fish.nix

@@ -765,6 +765,30 @@ in
           }) cfg.plugins
         );
       })
+      (
+        let
+          themes = lib.foldl (
+            themeList: plugin:
+            if lib.pathIsDirectory "${plugin.src}/themes" then
+              themeList ++ lib.filesystem.listFilesRecursive "${plugin.src}/themes"
+            else
+              themeList
+          ) [ ] cfg.plugins;
+        in
+        (mkIf (lib.length themes > 0) {
+          xdg.configFile = lib.mkMerge (
+            map (
+              theme:
+              let
+                basename = lib.last (builtins.split "/" (toString theme));
+              in
+              {
+                "fish/themes/${basename}".source = theme;
+              }
+            ) themes
+          );
+        })
+      )
     ]
   );
 }