Coding in a plugin vs theme query

[inscrewtable]

Hello,

I have some code that was written for my wordpress site that works with the minimal theme I have. The code is in the form of a Plug-In. The code is at the end of the post and what it does is put a red block around a menu item and open that item and close all the others.

If I go to a site like css-tricks.com will it have the information that I seek to be able to add a bit more functionality like have different colours for the Parent Child and Grandchild pages rather than them all being the same.

Basically I do not know if the CSS in the plugin references different CSS that is used in themes. It's all a bit bewildering. Like for example I tried putting some code I found into the plug in that would add the ability to change the Child colour, but it didn't do anything. Or for example I added this in to see what would happen, ie if it would do anything if I hovered, but nothing happened.

#navcontainer a:hover
{
background-color: #369;
color: #FFF;
}

Thanks.

nav.page-nav li { text-transform: uppercase; }
nav.page-nav ul.children li { text-transform: none; }
nav.page-nav li.current_page_item a {
    background: #9d0706;
    color: #fff;
    padding: 3px;
}
li.current_page_item ul.children li a {
	background: none;
    color: #555;
    padding: 0;
}
ul.children li.current_page_item ul.children li a {
	background: none;
    color: #555;
    padding: 0;
}
nav.page-nav .children { display: none; }
li.page_item_has_children.current_page_item > ul.children { display: block; }
.current_page_parent > ul.children { display: block; }
.current_page_ancestor > ul.children { display: block; }