You must be the happiest user on this board now.
You can play around all that you want. In case you go badly wrong just replace files or uninstall the messed up board and install a fresh one. Don´t forget that in case you fiddled around installing MODs that altered the database you should also drop the content of that database before reinstalling.
You can also have varios boards installed and working at the same time. Simply create a second, or third database with different names and install boards with different names on them.
Only thing you must watch is to use diferent cookie-names on every board you set up. This you can allready do on the install page of new boards. Allthough there´s some trickies to this.
But anyhow, when you mess up there´s no harm done.
I´m not gonna tell you how many testboards I messed up and re-installed before I started to get the hang of things.
To play around with backgrounds and forum layout you must first edit a few files to make your .css file work instead of your overall_header.tpl.
Once you can use your .css it´s easy to learn. Just edit something, see what changes and undo or continue making new edits.