Core: Use a constant for the default creator theme

Change-Id: I53ba63cf55fb6918d959b0d396175a3188d62bd9
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/coreconstants.h

@@ -198,6 +198,7 @@ const char SETTINGS_ID_MIMETYPES[] = "E.MimeTypes";
 const char SETTINGS_DEFAULTTEXTENCODING[] = "General/DefaultFileEncoding";
 
 const char SETTINGS_THEME[] = "Core/CreatorTheme";
+const char DEFAULT_THEME[] = "default";
 
 const char ALL_FILES_FILTER[]      = QT_TRANSLATE_NOOP("Core", "All Files (*)");