Change tab color

can’t tell from a picture … (a link to your site?)
however the tabs are likely to be graphics,
so you could start with giving a special class to the ‘forum’ list element, reproduce the graphic elements in crimson red, and then start styling it analog to the existing ones.

if you are planning to do more theme adaptation, downlowd the developer add-on for firefox. it is a very good tool to inspect html elements in your browser, and see which styles effect what on the page, and try changes online 😉

your style.css on line 136

first, you need to copy two graphic files, save them under the new name ‘forum_navtabl.png’ and ‘forum_navtabr.png’ resp. and turn them into red graphics:
/images/navtabl.png -> /images/forum_navtabl.png –
and
/images/navtabr.png -> /images/forum_navtabr.png –
these are the backgrounds of the tabs.

then you need to edit your line with the ‘forum’ link, (what it does, is to give this link the class=”forum” with which you can style in style.css), and remove the commentation:
<li class="page_item forum"><a href="http://www.animelovers-fansub.com/forum" title="Forum"><i>Forum</i></a></li>

next copy and adapt this part of style.css, so that these lines are added to style.css:

#nav li.forum {
	float: left;
	height: 30px;
	background: url(images/forum_navtabr.png) top right no-repeat;
	margin: 0 3px;
	white-space: nowrap;
}

#nav .page_item.forum a{
	color: #FFFFFF;
	display: block;
	background: url(images/forum_navtabl.png) top left no-repeat;
	text-decoration: none;
	padding: 0px 10px 0px 10px;
	font: bold 12px Arial, Helvetica, sans-serif;
	line-height: 30px;
}

#nav li:hover.forum {
	background-position: 100% -36px;

}
#nav .page_item.forum a:hover {
	background-position: 0px -36px;
	border: none;
}

this should be all, good luck 😉

well done so far 😉
if you look at the original images – they have shading and round corners.
you need to make the new images exactly the same, just with a different color. and keep the blue-background-rounded-corners.