Open components/com_virtuemart/themes/default/images folder and you can change the add to cart button to different colors (add-to-cart_color.gif). Or just replace it with your custom add to cart image button.

To remove text from add to cart button go to:

administrator/components/com_virtuemart/languages/common/english.php or used language instead of english.php

In the file look for 'PHPSHOP_CART_ADD_TO' => 'Add to Cart' You can change "Add to Cart" to anything you need or just leave it blank.

The background of the  box around the add to cart form is in the folder components/com_virtuemart/themes/default/ and edit the theme.css

Look for:

.vmCartContainer { /* Cart Container */
   width: auto;
   float: left;
   background: #F9B4D4;
   border: 1px solid #000;
   padding: 3px;
}

The default is background: lightblue. I usually change the background to the background color used on the site and I usually get rid of the border.