Here is the screenshot from chrome dev tools. Here I want to override .ui-menu class and want to increase its with to "20em".

Here is what I am trying to override it :
.menu.ui-menu{
width:20em;
}
I know my css is loading after the base css. Why its not taking the css supplied in custom css class?
UPDATE None of the suggestions provided below worked for me. I have added a div and provided id to div "menu". Now when I use following css , it works :
#menu {
width:20em;
background-color: yellow;
}
Here is the screenshot from console:

Still unable to increase width of menu that has class "ui-menu". Can someone please help in identifying the selector for "ui-menu" ?