Use #7a7a7a as the grey color in AIShell to meet the contrast requirement (#333)

Author: daxian-dbw

shell/AIShell.Kernel/Host.cs

@@ -419,7 +419,7 @@ public async Task<T> PromptForSelectionAsync<T>(string title, IEnumerable<T> cho
             .Title(title)
             .PageSize(10)
             .UseConverter(converter)
-            .MoreChoicesText("[grey](Move up and down to see more choices)[/]")
+            .MoreChoicesText("[#7a7a7a](Move up and down to see more choices)[/]")
             .AddChoices(choices);
 
         return await selection.ShowAsync(ansiConsole, cancellationToken).ConfigureAwait(false);
@@ -468,7 +468,7 @@ public async Task<string> PromptForTextAsync(string prompt, bool optional, IList
         RequireStdoutOrStderr(operation);
 
         IAnsiConsole ansiConsole = _outputRedirected ? _stderrConsole : AnsiConsole.Console;
-        string promptToUse = optional ? $"[grey][[Optional]][/] {prompt}" : prompt;
+        string promptToUse = optional ? $"[#7a7a7a][[Optional]][/] {prompt}" : prompt;
         var textPrompt = new TextPrompt<string>(promptToUse) { AllowEmpty = optional };
 
         if (choices?.Count > 0)

shell/AIShell.Kernel/Shell.cs

@@ -132,8 +132,10 @@ internal void ShowBanner()
     {
         string banner = _wrapper?.Banner is null ? "AI Shell" : _wrapper.Banner;
         string version = _wrapper?.Version is null ? _version : _wrapper.Version;
+
+        // The color '#7a7a7a' is most close to 'grey' but with high enough contrast to pass the contrast checker.
         Host.MarkupLine($"[bold]{banner.EscapeMarkup()}[/]")
-            .MarkupLine($"[grey]{version.EscapeMarkup()}[/]")
+            .MarkupLine($"[#7a7a7a]{version.EscapeMarkup()}[/]")
             .WriteLine();
     }
 
@@ -286,7 +288,7 @@ private void LoadAvailableAgents()
             chosenAgent ??= _agents.Count is 1
                 ? _agents[0]
                 : Host.PromptForSelectionAsync(
-                    title: "Welcome to AI Shell! We’re excited to have you explore our [bold]Public Preview[/]. Documentation is available at [link=https://aka.ms/AIShell-Docs]aka.ms/AIShell-Docs[/], and we’d love to hear your thoughts - share your feedback with us at [link=https://aka.ms/AIShell-Feedback]aka.ms/AIShell-Feedback[/].\n\n[orange1]Please select an AI [Blue]agent[/] to use[/]:\n[grey](You can switch to another agent later by typing [Blue]@<agent name>[/])[/]",
+                    title: "Welcome to AI Shell! We’re excited to have you explore our [bold]Public Preview[/]. Documentation is available at [link=https://aka.ms/AIShell-Docs]aka.ms/AIShell-Docs[/], and we’d love to hear your thoughts - share your feedback with us at [link=https://aka.ms/AIShell-Feedback]aka.ms/AIShell-Feedback[/].\n\n[orange1]Please select an AI [Blue]agent[/] to use[/]:\n[#7a7a7a](You can switch to another agent later by typing [Blue]@<agent name>[/])[/]",
                     choices: _agents,
                     converter: static a => a.Impl.Name)
                 .GetAwaiter().GetResult();