Certain CSS Auto Removed From Editor

Helloooo!
I am a freelance guy who helps some people with WordPress consultations. I know a moderate amount about coding and stuff, so what I mainly do is help clients with custom CSS and editing minor things in the php files of themes and widgets.

Anyway, the other day I’m sitting with a client and we’re trying to disable selecting/highlighting within a specific element on her webpage.
All of our custom CSS is in the built-in CSS Editor of WordPress so we can edit everything in one place. I typed out

#mbMedia, #mbMedia div { user-select: none; }

then when we hit save the “user-select: none;” line was removed (but not the name of the element). So we tried it again and same thing.

So I’m wondering how to disable selection/highlighting using CSS in WordPress and equally important, what all does WordPress automatically remove from their CSS Editor?

PS
I also tried
#mbMedia::selection, #mbMedia::selection div { background-color: transparent; }
which didn’t work either.

Here’s my client’s site:
https://organicconservative.net/