theme/grubber-darker: add grubber-darker theme, which is inspired by tsodings emacs theme

modules/plugins/theme/supported-themes.nix

@@ -37,6 +37,33 @@ in {
     '';
     styles = ["dark" "darker" "cool" "deep" "warm" "warmer"];
   };
+  gruber-darker = {
+    setup = _:
+    /*
+    lua
+    */
+    ''
+      require('gruber-darker').setup({
+        -- defaults
+        bold = true,
+        invert = {
+          signs = false,
+          tabline = false,
+          visual = false,
+        },
+        italic = {
+          strings = true,
+          comments = true,
+          operators = false,
+          folds = true,
+        },
+        undercurl = true,
+        underline = true,
+      })
+      vim.cmd('colorscheme gruber-darker')
+    '';
+    styles = ["dark"];
+  };
 
   tokyonight = {
     setup = {