Why isn't my custom theme working?
Upgrade to Carbonize v1.1 to fix this issue.
Changes in the AirPlay protocol forced me to discontinue Porthole early 2017. As of 2018 Apple pushed more AirPlay updates that make Porthole incompatible with the latest firmware of the Apple TV and the AirPort Express.
Luckily there are a few alternatives. If you use a recent version of macOS, you can actually go into your Sound preferences and select your AirPlay device there. (Use this shortcut to make it almost as convenient as Porthole.)
If you want to stream to multiple AirPlay devices at the same time, you can either have a look at Rogue Amoeba’s Airfoil or wait until the next installment of macOS and iOS are released (probably by the end of September). iOS 12 and macOS Mojave have support for AirPlay 2, which allows you to stream to multiple devices out of the box!