Where to find the css lines I want to change?

I wanted to change the color of the sidebar widget templates in my theme. I wrote to the support people there and they told be to put the color I wanted into this line:
.sidebar .widget-title { color: #333; }
I did and it worked perfectly. I chose the Add To option, of course, in the Edit CSS screen in appearances.

There are a bunch of little things I want to change, for example, some font sizes, type colors in other areas, etc.

I’m assuming, perhaps falsely, somewhere in the theme’s CSS there’s the line that I changed. It’s easy to read and I would have known that this code is what I wanted.

So my question is, where can I find a theme’s CSS so that I can examine it to find the lines/syntax for things I want to change, for example, the color of the type in the top menu.

Of course I wouldn’t change anything in the “real” CSS file, but I’d add a couple of things in the CSS editor the way I did with the widget headline color. If what I put in didn’t work, I could just take it out again, so I don’t think I’d mess anything up.

So, if anyone can tell where to find the file, I wouldn’t have to write theme support every time I wanted to make some little changes like this.

Thanks,
Martin