If you haven’t made any changes to your beta program settings for Boundess, then you’re definitely on the current stable C++ build (C++ is just the programming language used to create the game).
If you want to try the current test branch of the game, to see if you still get the sky glitch, you can do so from your Steam library…
- Right-click on Boundless in the games menu (on the left), then select ‘Properties’.
- In the Properties window, click on the BETAS tabs at the top.
- In the drop down, select “testing - Unstable new feature development”, then click Close.
Steam will then download the current test build of the game (it’ll just show it as updating in the normal way).
Click Play as you normally would and you’re now playing the latest test version (and hopefully won’t have a glitchy sky!).
If you want to get back to the stable build, simply repeat the process, but select “None - Opt out of all beta programs”, and you’re back in the land of stable releases 