Custom CSS

I'm trying to add custom css and after clicking update I get the following error message (I'm just trying to change the color of the SELECT OPTIONS button and text):

Not Acceptable!
An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.

Here's the CSS I'm trying to add:

.woocommerce #respond input#submit, .woocommerce a.button, .woocommerce button.button, .woocommerce input.button {
font-size: 100%;
margin: 0;
line-height: 1;
cursor: pointer;
position: relative;
text-decoration: none;
overflow: visible;
padding: .618em 1em;
font-weight: 700;
border-radius: 3px;
left: auto;
color: #515151;
background-color: #ebe9eb;
border: 0;
display: inline-block;
background-image: none;
box-shadow: none;
text-shadow: none;
}