Sencha - Add bottom header border to a panel

I have a panel (extjs panel) and I need to draw only the bottom border of the panel header.

I checked out using firebug to my web application theme that there is a css property that allow to draw a bottom border of a panel header:

.x-docked-top{
   border-bottom-width: 0 !important;  // neptune-theme.css
}

As I remarked, this property is set via neptune theming template... so I need to set this property to '1' like this:

.x-docked-top{
       border-bottom-width: 1 !important;  // myCustomCss.css
}

The problem is that I don't know why if I write in my myCustomCss.css file the above code, it doesn't work and nothing happen. Maybe the !important feature used in neptune-theme.css has major priority?.

Does anyone know how to fix it?