Doc: Update info about default help viewer backend

It is now litehtml. Update screenshots to show the help using
the litehtml backend and to use the Design Dark theme.

Task-number: QTCREATORBUG-24529
Change-Id: I7e230e7e0a154161f525ba37d0965ac1c50d2ea6
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

doc/qtcreator/src/howto/creator-help.qdoc

@@ -75,24 +75,11 @@
     To switch to the editor context when you close the last help page, select
     the \uicontrol {Return to editor on closing the last page} check box.
 
-    \section1 Selecting the Help Viewer Backend
-
     The help viewer backend determines the style sheet that is used to display
-    the help files. A help viewer backend that is based on the \l {QTextBrowser}
-    is used by default. However, it has several issues that have been fixed when
-    using litehtml as the help viewer backend.
-
-    On \macos, you can also use a help viewer backend that is based on the
-    system WebView component.
-
-    To switch the help viewer backend:
-
-    \list 1
-        \li Select \uicontrol Tools > \uicontrol Options > \uicontrol Help >
-        \uicontrol General.
-        \li In the \uicontrol {Viewer backend} field, select a viewer backend.
-        \li Reload the help page to take the new backend to use.
-    \endlist
+    the help files. The default help viewer backend that is based on litehtml
+    is recommended for viewing Qt documentation. You can choose another help
+    viewer backend in the \uicontrol {Viewer backend} field. To take the new
+    backend to use, reload the help page.
 
     \section1 Viewing Function Tooltips