created impress-common.css (#760)

Co-authored-by: Ingrid-Regina Vähi <ingrid.vahi@adm.ee>

examples/markdown/index.html

@@ -15,7 +15,8 @@
     <meta name="author" content="Henrik Ingo" />
     <link rel="stylesheet" href="../../extras/highlight/styles/github.css">
 
-        
+    <!-- Common styles for example presentations -->  
+    <link href="..\..\css\impress-common.css" rel="stylesheet" />  
     <!--
         Styles specific for this example presentation. 
     -->