You need to change the last one to yes correct to use the forum style. Now the css must exist on disk so if your style is null then we need to figure out whats wrong as in is the css not being found etc.
I see your issue see your phpBB style is quite custom. Usually in phpBB the style name is the same as the directory name of the style in your case Iris in the phpBB2 templates directory. Therefore if you save the contents of the file below as Iris.css and upload it to your Iris directory under templates you should be fine to turn on the style. Your special Iris css file is named style1.css which is your issue.