Modifying thumbnail size

I'm trying to create an Exhibit Builder layout that displays a gallery with rows of 6 100-pixel thumbnails rather than 3 200-pixel thumbnails.

When I modify the CSS to:

#thumbnail {
    width: 100px;

it downsizes the images, but they're all in a single column. It seems to be restricting the size of the whole gallery rather than just the individual images.

Is there a way to achieve my desired result within the CSS/PHP for the layout, or would it be better to create a new thumbnail type entirely? (And if so, where in the file structure would I do that?)

Never mind. It was as simple as calling them exhibit-item rather than thumbnail.

I swear I'll figure this stuff out eventually.