On my website at http://www.cornetsdegroot.com, I would like to change the color of the <hr> below the breadcrumb to black; it is #cccccc (gray) now. I would also like to add more space above and below the <hr>. Can anyone please tell me where should I do this?
Many thanks,
Rutger

look at the source of the page and identify the HTML elements. With that you can construce a CSS Selector and set of rules(styles) that you put in teh CSS Inserts area

Yes, that is the way to go about it but I can't figure out where the HR originates. I even installed Firebug and still I'm unsure. Worst thing is, I think I put it there myself... But where is there?

ok, this is created by a plugin. If you look at the generated page you see

<div class="breadcrumb">
<!-- Breadcrumb NavXT 3.4.1 -->
De open ruimte</div>
		<hr>
<div class="post-3 page hentry category-geen-categorie post" id="post-3">

so it looks like the <hr> is tossed out ther outside the breadcrumb <div>
you can apply CSS to the breadcrum using '.breadcrumb' and since the <hr> is the only occurance on the page you could code a CSS Selector or it as

hr {.....}

Thanks again juggledad, you're a great help! All pointers you have given me have been spot-on and accurate. I'm becoming increasingly excited about WP and the support I've received so far!