Update doc

Author: dvoituron

examples/Demo/FluentUI.Demo.Client/Documentation/Components/Overlay/Examples/OverlayHtmlElement.razor

@@ -14,6 +14,7 @@
     <fluent-overlay id="id1"
                     dialog-style="background: transparent;"
                     dialog-class="my-class"
+                    background="color-mix(in srgb, var(--colorBrandBackground) 20%, transparent)"
                     fullscreen="@FullScreen"
                     interactive="@Interactive"
                     close-mode="@CloseMode">