Hey there!
We've seen this issue occassionally in systems that don't fully support the renderer the engine uses.
I have a couple of suggestions:
1. Firstly, try pressing shift + g in game and changing the renderer. There should be a few options, and you'll have to restart between each change, so try all of the options available.
2. If that doesn't work, try going to your graphics card manufacturers website and make sure there's not an update for your drivers. We've seen this behaviour very rarely with out of date drivers.
3. If you're using a computer with multiple graphics cards, make sure in your OS settings that the game is running using your dedicated graphics cards. Sometimes in two graphic card setups, we've seen computers assign renpy games to the non-dedicated/non-gaming one.