correct invalid theme name in old config file

src/gajim.py

@@ -1059,6 +1059,9 @@ class Interface:
 				gajim.config.add_per('statusmsg', msg)
 				gajim.config.set_per('statusmsg', msg, 'message', gajim.config.statusmsg_default[msg])
 		#add default themes if there is not in the config file
+		theme = gajim.config.get('roster_theme')
+		if not theme in gajim.config.get_per('themes'):
+			gajim.config.set('roster_theme', 'green')
 		if len(gajim.config.get_per('themes')) == 0:
 			d = ['accounttextcolor', 'accountbgcolor', 'accountfont',
 				'grouptextcolor', 'groupbgcolor', 'groupfont', 'contacttextcolor',