changes vi mode appearance in fish shell

modules/fish/default.nix

@@ -95,6 +95,29 @@
         # Open file in neovim
         nvim $fullpath
       '';
+      fish_mode_prompt = ''
+        switch $fish_bind_mode
+          case default
+            set_color --bold blue
+            echo 'N'
+          case insert
+            set_color --bold green
+            echo 'I'
+          case replace_one
+            set_color --bold green
+            echo 'R'
+          case replace
+            set_color --bold bryellow
+            echo 'R'
+          case visual
+            set_color --bold brmagenta
+            echo 'V'
+          case '*'
+            set_color --bold red
+            echo '?'
+        end
+        set_color normal
+      '';
     };
 
     interactiveShellInit = ''