Yes you can change the color to whatever you want with css. Take a look in your css file in the Optimal Blue section. The only thing that you won’t be able to change with css are the images with gradients. Since they are images you would have to either recreate the images or remove them and replace them with a css color. The bottom of the slider with the gradient is an image – http://www.hiltoncopy.com/wp-content/themes/optimal/images/dark-blue/slider-bg.png but the header is css.
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