If classes or ids don't exist in your theme you can just create them. It is likely that any style you see is being created in the parent theme (ususally Standard). You don't need or want to edit this though, you can just overule it by including a similar rule in one of your own theme's files like so.
.current .side { background-color: red; }
where .current specifies only the selected topic.
That's a very clever solution. I'm impressed
Did the link and the button work as expected before you added the new header? The middle section of kubrick actually overlaps the navigation bar by about 20px. This should go behind the link trail and the button, but in your screenshot it appears to be in front of them, which would explain why you can't click the link, or see half the button.
You could probably force the z-order in the CSS (if that means nothing to you, here's a brief introduction to z-order) or I could take a look at the site if it's publicly accessible somewhere. I'm surprised that it works one way in some browsers, and another way in others, but I shouldn't be really.
Regarding the image, what method have you used to update the header?
If you used the last method then you need to be aware that the standard image includes a few pixels of grey border to blend it in with the background. You could simply load that original image into Photoshop and replace the central blue area with your own header while leaving that border in place.
Alternatively the other image uploading instructions should automatically crop and centre the image for you, this generally works quite well, but will probably clash with the border you've added to the image, try changing it to a plain colored background and trying one of the first two upload methods.
I think just adding the following line to the end of one of the .css files in your current theme should do the trick.
.highlight { background-color:#AAFFAA; }
I just wanted to say:
microformats: yay! I'd not seen anything about forum/blog posts but stuff I'd looked at previously (identity, calendar and reviews stuff) is very interesting.
you can link to individual Moodle posts if you know how, though it isn't exposed to the casual user e.g your post is:
http://moodle.org/mod/forum/discuss.php?d=27338#1390030
I got that by clicking reply and then stealing the id from the URL. Now that I've replied to your post you can do it more easily by clicking Show parent in my post. I'm not sure if this should be done (is the topic more important than the individual post?) but it currently it can if you know how.