Firebug (right click - inspect element) will show you what tags are controlling the style element - then you need to find that tag in the CSS files for your theme. If you are using prosilver, the CSS files are named according to what they contain, and well commented. You can search for text in local copies of the CSS files to find where a tag is defined. I use grep on linux, but there are Windows tools for finding text in a group of files too (one of them being Windows Grep !).
prosilver CSS is quite complex, but you'll get used to it.
Did you find the Styles Development and Discussion Forum
That's a good place to start understanding Styles in phpBB3.