Yes, it's quite easy to adjust text colors and so forth. The green images will either have to be overwritten with a new color or perhaps tinted in a image program. Unfortunately, you'll have to adjust the text colors via the actual stylesheet.css. I'm thinking you're referring to admin stylesheet control as like perhaps SMF forum program or so forth, no, Zen Cart doesn't currently offer that.
You cannot "overlay" a template on top of another. You must upload a template into the directory on it's own. Following the directory includes/templates, you will note how there are two template folders named "classic" and "template_default". You will put your custom template folder along side of them. A simple method is to make a desktop copy of the green Classic template folder/sub folders. There is one file named 'template_info.php' and you must adjust the template name for Zen Cart to acknowledge the new edition. If you open the noted file in a text editor, find;
Code:
$template_name = 'Classic Contemporary Green';
Replace above with;
Code:
$template_name = 'Your New Template Name';
Be sure to leave the surrounding quote marks intact.
Then, rename the "classic" template folder to your new template name and re-upload it as a new template base.
You can also upload a contributed template to your liking here;
http://(sorry, site offline)/