WordPress Blog posts navigation

Hi Folks

I have the following navigation on my website:

Home | About | Skills | Portfolio | Blog

Here is the html:

<nav>
	 <div class="menu-main-navigation-container">
<ul id="menu-main-navigation" class="menu">
<li id="menu-item-42" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-42"><a href="http://localhost:8888/paulkenyon/home/">Home</a></li>
<li id="menu-item-41" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-41"><a href="http://localhost:8888/paulkenyon/about/">About</a></li>
<li id="menu-item-45" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-45"><a href="http://localhost:8888/paulkenyon/skills/">Skills</a></li>
<li id="menu-item-56" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-56"><a href="http://localhost:8888/paulkenyon/portfolio/">Portfolio</a></li>
<li id="menu-item-52" class="menu-item menu-item-type-post_type menu-item-object-page current-menu-item page_item page-item-50 current_page_item menu-item-52"><a href="http://localhost:8888/paulkenyon/blog/">Blog</a>
</li>
</ul></div>			</nav>

When the user goes to a page, eg) Blog the link is highlighted with a background image.

Here is the CSS:

header ul li.current-menu-item a,
header ul li a:hover
{
-moz-border-radius: 4px;
-webkit-border-radius: 4px;
-o-border-radius: 4px;
-ms-border-radius: 4px;
-khtml-border-radius: 4px;
border-radius: 4px;
background-color: rgba(0, 0, 0, 0.042);
border: 1px solid rgba(0, 0, 0, 0.15);
margin: 0;
border-image: initial;
padding-left: 5px;
padding-right: 5px;
padding-top: 3px;
padding-bottom: 3px;
color: #939393;
}

header ul li.selected {
-moz-border-radius: 4px;
-webkit-border-radius: 4px;
-o-border-radius: 4px;
-ms-border-radius: 4px;
-khtml-border-radius: 4px;
border-radius: 4px;
background-color: rgba(0, 0, 0, 0.042);
border: 1px solid rgba(0, 0, 0, 0.15);
margin-top: -3px;
border-image: initial;
padding-left: 0px;
padding-right: 0px;
padding-top: 3px;
padding-bottom: 3px;
color: #939393;
}

What I am trying to work out is when the user clicks on a post or categories page or archives page I want the ‘Blog’ anchor link to remain highlighted with a different background.

At the moment when you click on a post the selected active link doesn’t show.

Hope the above makes sense?! Im not sure if there is a function I can use or if its simply the css?

Any ideas any one?

Thanks in advance

Paul