The IAP lets you change fonts, tint color for buttons and the dark mode theme is unlocked permanently. I didn't want to lock the dark mode for free users - so free users can still enable dark mode but it's on a per session basis. When the app is relaunched (closed from the app switcher or killed by the OS), it defaults to the light theme for free users. I thought this was a good balance for free users and also have the ability to use dark mode permanently.
That's a very clever way to let users test drive dark mode.
There was a HN client posted a couple weeks ago, but virtually all settings and customization was locked behind IAP. Without being able to really try the app at all, I just uninstalled it.
Would be nice to be able to try out the other settings such as don't like this as well so we easier could make an assessment of the app, I'd gladly pay if it solves the largest quriks/issues I have with the app I'm currently using, but I'd like to try it out properly first.