How I add CSS element?

I'm new to this business, I use nicepage because it is comfortable to use. But there is one thing I want and cannot do. I made one box and I want to blur this box. There is no such option. However, as a result of my research, I learned that I can do this with CSS. I found a blur css code from the internet and pasted it in "Edit CSS:" but nothing changed. How can I set this blur to affect the box. Please help.