Buttons
There are two button types: "Add to cart" button (the "Checkout" button in the cart page is the same) and any other button.
.PSD source for buttons: Download
There are images in the [YourMagentoDir] /skin / frontend / default / [bluescale] / images / folder:
add_btn_split.gif = "Add to cart" button on the product page
blue_btn_split.gif = blue buttons in the catalog
r_arr_btn_split.gif = checkout button
l_arr_btn_split.gif = button with left arrow on the checkout page
In order to change button text color open [YourMagentoDir] / skin / frontend / default / [bluescale] / css / skin.css and find the following code.
"Add to cart" button, line 1352
Blue button, line 1215
Checkout button, line 1236
Feel free to make changes.
Background
In order to change the background open /[YourMagentoDir]/skin/frontend/default/[YourSkin]/css/skin.css, and add the following code to the end of skin.css file.