I know that the new themes are supposed to use CSS but as far as I know we don't have easy access to the CSS code.
The best way to accopmplish what you want would be to simply change the height of your fields using the Position tab on the Inspector.
You can select multiple fields and change them all at once as long as they are all the same height to begin with.
Obviously if you shrink your fields down too far you will have issues with the text displaying properly.
Unfortunately any new fields you create will conform to the theme so you will have to change them manually as well.
If you are serious about actually editing the CSS file for the theme I believe that can be done but I have not tried it.
If you Google "Edit Filemaker 12 CSS" or "Make your own Filemaker 12 Themes" I know some people are already playing with this and you might find a tutorial.
Which really raises a question...
Why does the layout default to 612 pixels wide and why in a world where preferences have been around for 25 years or more does Filemaker not employ them properly for use developers. Why, why, why...
OK, it is really nice to be able to create a layout and switch to another choic of colors, etc. and watch it all change at once. Except for selecting a nother layout style which means the fields now are too large... :)