I installed a boot screen to substitute the black boot screen that comes with the Mac. Let's say now it's unbelievably laggy and I want to switch back.

I had an app help me, called "Boot Buddy". I installed it through Chrome. I open it and choose a design that I like, so I hit "Apply changes" and a message of permissions, macOS, et cetera appears. I read it, but still click on "yes". Now I regret it, and I want to know if there is any way I can change back, or remove the permission to modify my Mac. Thanks in advance.