Why does the Custom CSS plugin doesn’t seem to work anymore?

When I enter these CSS to remove the drop shadow of the menu:

.main-navigation.sticky {
-webkit-box-shadow: none;
-moz-box-shadow: none;
box-shadow: none;
}

And save CSS, the modifications don’t appear on the website. Then I made some test and there are no modifications to the CSS that work.

Do you have a solution?
Thank