StartPage implement custom CSS preference

src/Mod/Start/StartPage/StartPage.html

@@ -6,7 +6,8 @@
     <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
     <script>JS</script>
     <style>
-        CSS
+        DEFAULT_CSS
+        CUSTOM_CSS
     </style>
     <!--QSS-->
 </head>

src/Mod/Start/StartPage/StartPage.py

@@ -352,7 +352,8 @@ def handle():
     with open(css_filename, 'r') as f:
         CSS = f.read()
     HTML = HTML.replace("JS",JS)
-    HTML = HTML.replace("CSS",CSS)
+    HTML = HTML.replace("DEFAULT_CSS",CSS)
+    HTML = HTML.replace("CUSTOM_CSS",FreeCAD.ParamGet("User parameter:BaseApp/Preferences/Mod/Start").GetString("CustomCSS","").replace("\n",""))
 
     # set the language