Best way to adjust side bar width

hi on the type of layout you can see at

How can i adjust the sidebar where the nav menu is on the left hand side,, to be wider?

Currently I can see this css rule

.html_header_sidebar #top #header {
width: 300px;
}

But if I want to increase it to 400px, I tried adjust the above css to

.html_header_sidebar #top #header {
width: 400px;
}

But it doesn’t really resize it, it more over laps the content and there is no border visible

Can you advise best method to do this?