Look I love FOSS, but this mentality that using anything except for FOSS is dumb. An incredible amount of time, money, and effort goes into building an app like Boost and the developer has every right to keep it closed source and charge for it and you have every right not to use it.
Many people are more than willing to pay for great software and others are happy to give up some privacy to get it for free. That’s their choice.
If you have dark mode enabled in your android display settings then you can use the dynamic color option in boost to inherit amoled black from there.