Altering 'Syndney' WordPress Theme Header

Hi Everyone,

I’m very new to WordPress but after setting everything up I’m now looking at altering the default appearance of the ‘Sydney’ theme that I’ve installed. My first issue is I want to reduce the height of the scrolling header slider. I’ve installed Firebug and have highlighted and know what code needs to be changed to get the outcome I would like, but to be honest I have no idea how to locate what file these are stored in. Below is the code that links to the header image I would like to alter:

<div id=”slideshow” class=”header-slider” data-speed=”4000″ style=”position: relative; overflow: hidden; width: 100%; height: 531px;”>
<div class=”slides-control” style=”position: relative; height: 100%; width: 5709px; left: -1903px;”>
<div class=”slides-container” style=”margin: 0px; padding: 0px; list-style: outside none none; position: relative; height: 100%;”>
<div class=”slide-item” style=”background-image: url(“http://localhost/ftcg.co.uk/wordpress/wp-content/themes/sydney/images/1.jpg&#8221;); display: block; left: 1903px; position: absolute; overflow: hidden; height: 100%; width: 1903px; top: 0px; z-index: 2; opacity: 1; background-position: 50% 0px;”>

Many Thanks