files important for theme building?

[Borsa]

Hallo,
i'm just building my own theme (or changing default one), but besides css files (global.css) i need to make some changes in another code (I still mean layout). In joomla its just about index.php in the template folder, in prestashop template folder i've found *.tpl files, but when i try to change for example header.tpl, thats behave pretty strange, maybe i can say its ignore all of new elements which i'm trying to add.
(in global.css i defined #mainheader, but when i add div id='mainheader' to header.tpl, its ignoring defined size, borders, colors, and it shows me only text inside this div)
Is there any general file where ill be able to change layout?
Thanks, Borsa