sfuaad - You could use the context objects for this, such as
<#if coreNode.id == "cat_1">
//style for cat_1
<#elseif coreNode.id == "cat_2">
//style for cat_2
<#elseif coreNode.id == "cat_n">
//style for cat_n
</#if>
I know this might be difficult to scale, but unless you don't have unique classes for each the category in header, you cant't do it via CSS alone, and unfortunately, this is the case. Also, even in case of CSS, you'll have to update CSS for each new category, and same goes with this code So, you can include this code in header, where it will execute on each category page.
Hope this helps.
