By default the app uses the .btn class that most Shopify themes support. So the idea being that if your store has a specific look for your buttons the app uses the same one.
However, sometimes themes don't have a style for the .btn class so it ends up just looking like a generic link. It's each to fix this though. You just need to inject any css you want to the #customButton id. This ID is only on that donations button, so it won't effect the other buttons on your website.
Here's a video showing more details: