Prosilver and sub silver are probably the two clearest themes for iphone viewing. You might still need to zoom the page out a bit, but I find them the clearest on the phone.
There are things like Tapatalk that will work with (possibly only certain versions of) phpBB, but that's a plug-in that needs installing and configuring on the site itself, so that's not going to work for here. And there are very good reasons why a forum or site owner wouldn't want to install it. (There are obviously reasons why they would, otherwise it wouldn't be being used so much.) It's also sodding annoying in it's default site config by shoving irritating pop-ups on your mobile browser telling you there's an app you can read the forum on. (As you might be able to tell, I'm not a fan
![Wink ;)](./images/smilies/icon_e_wink.gif)
)
So I'd go for changing the theme
![Smile :)](./images/smilies/icon_e_smile.gif)