The layout works only when the tool is installed under root directory like http://a.com / however, if you want to use it under URLs like http://a.com /opcachegui or http://a.com /tools/opcachegui the layout is terrible because all JS/CSS files couldn't be found.
I was going to fix it (and send a pull request) by myself; however, I noticed that there were many files involved so it would be better to leave it to the author since the author may prefer to fix it in a way different from what I might choose.
It's really a nice tool. Thanks for your effort.