Enhance the style of the checkboxes for multi-selection (#1244)

* Enhance the checkboxes' style for multi-selection by applying the selected style, if available, to the checkboxes currently selected.

src/Spectre.Console/Prompts/List/ListPromptConstants.cs

@@ -9,4 +9,15 @@ internal sealed class ListPromptConstants
     public const string InstructionsMarkup = "[grey](Press <space> to select, <enter> to accept)[/]";
     public const string MoreChoicesMarkup = "[grey](Move up and down to reveal more choices)[/]";
     public const string SearchPlaceholderMarkup = "[grey](Type to search)[/]";
+
+    public static string GetSelectedCheckbox(bool isGroup, SelectionMode mode, Style? style = null)
+    {
+        if (style != null)
+        {
+            return "[[" + $"[{style.ToMarkup()}]X[/]" + "]]";
+        }
+
+        return isGroup && mode == SelectionMode.Leaf
+            ? GroupSelectedCheckbox : SelectedCheckbox;
+    }
 }

src/Spectre.Console/Prompts/MultiSelectionPrompt.cs

@@ -257,8 +257,7 @@ public sealed class MultiSelectionPrompt<T> : IPrompt<List<T>>, IListPromptStrat
             }
 
             var checkbox = item.Node.IsSelected
-                ? (item.Node.IsGroup && Mode == SelectionMode.Leaf
-                    ? ListPromptConstants.GroupSelectedCheckbox : ListPromptConstants.SelectedCheckbox)
+                ? ListPromptConstants.GetSelectedCheckbox(item.Node.IsGroup, Mode, HighlightStyle)
                 : ListPromptConstants.Checkbox;
 
             grid.AddRow(new Markup(indent + prompt + " " + checkbox + " " + text, style));