Don't have a lot of experience with SMF themes, but this looks like a nice place to start learning:

http://custom.simplemachines.org/themes/

I could certainly help you but which theme are you currently using?

OK Bluewaves, thanks for that, I think you've got me on the right track at least, but when I looked at that file I got a bit nervous about tampering with it!
I don't have a logo or image as such, to replace the smflogo.gif with. I was wondering if it would be possible to put the same header that I have on my main site, across the top of the forum?
If I want to put Google ads somewhere on the forum, would I do that in this same file also? Is this a bad idea?

OK, I managed to get rid of the smf logo in favour of something more relevant to my site (I put a photo in there for now, since I don't have my own logo). And because I couldn't figure out how to change the banner to something like what I have on my site's pages, I changed my pages to look more like the forum!  I think the site looks better for it actually. Necessity is the mother of invention, after all!
Now (Bluewaves or anyone else), is it possible for me to insert some Google ads, for example in the space between the header and the message boards? If it's possible to do this in the same file (index.template.php) can you point out to me where exactly? Oh yes, and how? 

http://infotorremolinos.com/forum/