How do I modify existing themes in 12.04?

Solution 1:

Just go to /usr/share/themes and you will find the theme files.

For instance if you want to edit the Ambiance theme then get into the Ambiance theme folder and you'll find all the CSS and settings files, its really very basic it all consisits of CSS styles and color codes with some images.

Solution 2:

The easiest way I know of is to install gnome-tweak-tool for any adjustments such as cursors, fonts and icon packs. This allows you to adjust these options on-the-fly and works with both Unity and Gnome Shell (although some of the features, like Shell Extensions, expect Gnome Shell to be installed).