Depends on your theme. If you are still using the Simplicity theme, you could use this.

.ow_site_panel {
     border-color: your color;
}
You can do a search for "css border" to see other settings like border-style, border-size.

its not changing border color 

The site panel is the one at the top of the page that holds the nav menu, and console items. It actually didn't have a border. Is this what you are talking about?

yes 

Can I get your site url. Using inspect element and seeing it is sometimes easier.

You could try this.
.ow_site_panel {
    border: 2px solid black;
}

This will set a border width of 2px that is solid, and black. If it works you can modify it as needed for color, and size.

thankx its working 

its showing border on both sides uper and lower is there any way to show on lower site only thankx 

Yes. using border sets automatically sets all the way around. change it to this.

.ow_site_panel {
    border-bottom: 2px;
    border-style: solid;
    border-color: black;
}

That should do it.

its working thankx :) 

Yeah. Success. I knew I had the right element. I had to add the border to my theme a while back. That's how I knew it didn't have one. At least it didn't then.

Thanks Darryl