The new version, 2.6, brings support for the dark interface and will automatically appear if you have the darker theme enabled in your iOS display settings. If you’re using the standard light theme, you’ll notice no difference. There is no option to turn dark mode on or off for TestFlight specifically, yet.
Update – we can confirm you need access to the TestFlight Beta. You can request access from Apple on the Developer website, but you may not get it. To see dark mode, you must be running iOS 13 and the TestFlight Beta
If you’re running iOS or iPadOS 13, you get the beta version of TestFlight, so check that for a new update. Otherwise, you’ll need to wait until September when Dark Theme support comes to the public version. Yes, it’s quite confusing!