The site configuration panel is the first screen you get when you load the CMS. It is also known as the 'SiteConfig' panel. You can also get into the panel by clicking the root item (in bold) of your SiteTree on the left.
Thanks, willr, but that's not what I see. The screen is blank. I'm new as well. I've inherited a site that was created several years ago by someone who is not with the company any longer and I'm at a loss trying to figure out how this was set up. I can't find the code or the theme or anything that will let me make changes to the header or footer. I'm stumped. I hesitate to ask because there has to be something really obvious I'm missing. I don't know what I don't know, I suppose.
...also, I'm using 2.3.2 so maybe that's my problem?
Earlier versions of SS didn’t have an option to change the theme in the cms, so if you are not seeing the same theme dropdown box that appears in Will’s screenshot you will have to change it manually in the config file.
As for where the code is hidden, it depends. The php is in mysite/code, and the includes, css and html files are found under themes/mytheme/… It’s worth noting that in SilverStripe the html is saved as an “ss” file.
Your best bet is to do a quick run through the tutorials. They are fairly straightforward and should answer most of the questions that pop up as you try to update the site. If you come across something that doesn’t make sense just post a question in the forum. There may be some lag time because of time differences, but someone will eventually answer.