Custom CSS doesn't work in Unbounce (gif video shows issue)

Hi, I’m new to Unbounce but no stranger to CSS.

I’m trying to add custom CSS to my page, but it isn’t working.
Here’s a short 20 second screencast GIF I made illustrating my problem.

In this Gif you see me:

• Open a stylesheet in my Unbounce editor and paste a CSS rule (that changes the background color of dropdown boxes)

• The CSS does not work

• Then you’ll see me open my browser inspector, paste the rule in there, and it works

I’ve tried adding the !important declaration just for testing and it doesn’t help.
I’ve also tried the CSS with and without tags and it doesn’t work.

I’m sure I’m missing something native to Unbounce, but I don’t know what.