Band-aid fix for deprecate gtk_css_provider_get_default

by gtk_css_provider_new

Don't hesitate to revert this one but only if there's a patch
to fix it.
Change-Id: Id91fefb689326a31e029a788bd3729e670fe3100
Reviewed-on: https://gerrit.libreoffice.org/60915Reviewed-by: Julien Nabet <serval2412@yahoo.fr>
Tested-by: Julien Nabet <serval2412@yahoo.fr>

libreofficekit/qa/gtktiledviewer/gtv-helpers.cxx

@@ -105,7 +105,7 @@ GtkWidget* GtvHelpers::createCommentBox(const boost::property_tree::ptree& aComm
     if (aComment.get("parent", -1) > 0)
     {
         GtkStyleContext* pStyleContext = gtk_widget_get_style_context(pCommentVBox);
-        GtkCssProvider* pCssProvider = gtk_css_provider_get_default();
+        GtkCssProvider* pCssProvider = gtk_css_provider_new();
         gtk_style_context_add_class(pStyleContext, "commentbox");
         gtk_style_context_add_provider(pStyleContext, GTK_STYLE_PROVIDER(pCssProvider), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
         gtk_css_provider_load_from_data(pCssProvider, ".commentbox {background-color: lightgreen;}", -1, nullptr);