ah now I am getting you….. its not the styling your really after its adding new html to a file so that you can change your theme so that you can then style for the new html?…..
well, and this is the real bummer. the content you want to put in could be anywhere in your huge list of php files for wordpress.
but there are clues…. a bit of detective work and you will find where to add things to.
your main files are thte ones in your theme, so you might be able to add some html there and it changes.
BUT, what if your using dynamic sidebars?
you will have to find those php files that deal with it.
so as said above, a php file is kind of like the building blocks of the site, it brings in the first bit of html and so on (I know it does not only work on html, but its easier this way).
open your index.php and view the source in your browser first, find where you want to put your new code; is it the sidebar? then go to the sidebar.php and find where abouts you want to add your new html, remember php echos out html, so if you stick to what is around you should not bork things. is it in a dynamic sidebar or widget?
then if you look at the php you should see what file it is trying to include, then you have to do some digging to find that php file. if you find it, then open it up and look carefully at the php and html (i prefer to look for the html first, there are more clues) then find the part you want to change/amend and hopefully it all works.
good luck, it took me about 3 months to figure all of my problems out in wordpress