How to style presets- help please

Hi,

I want to style (mainly float) my presets in a series of views I created.

Using Firebug I can see that the class="imagecache imagecache-story_image_thumbnail".

So, would the following code work (I am a CSS novice - it doesn't seem to!):

.imagecache imagecache-story_image_thumbnail {
display: block;
float: right;
}

And what file should I put it in - my theme's style.css?

Help very much appreciated!

Thank you.