Will my style changes I’ve made change when I update theme?

Hey guys. I am currently using wordpress, and I'm a newb to it.
I've made some style changes by adding custom css to my theme, to make images, buttons etc appear how I want them to.
My questions is will this be overwritten when I update the theme?
– is there a way to update features of theme like security, functionality etc without touching the style?

Cheers guys