Fix focus box around color schemes combo box (#12439)

The focus box around the color schemes combo box was getting cut off, this change adds a small margin to the stackpanel to allow space for the focus box

## PR Checklist
* [x] Closes #12328

src/cascadia/TerminalSettingsEditor/ColorSchemes.xaml

@@ -99,7 +99,8 @@
                     Spacing="24">
 
             <!--  Select Color and Add New Button  -->
-            <StackPanel Orientation="Horizontal">
+            <StackPanel Margin="0,5,0,0"
+                        Orientation="Horizontal">
 
                 <StackPanel Orientation="Horizontal"
                             Visibility="{x:Bind local:Converters.InvertedBooleanToVisibility(IsRenaming), Mode=OneWay}">