Way to override CSS with saving theme settings functionality?

In at-commerce theme i found mistake in CSS3 markup in top and bottom color for buttons. Below gradient for Firefox is not equal to Webkit's, it should be 180deg, instead -90deg. I can override it with my CSS but want to save setting functionality to choose top and bottom color for buttons.
Also i suggest -o-linear-gradient(top, #A2D3E9, #7ABEDF); to add.
So, what is the better way to override possible mistakes in core theme while saving settings?

input[type="submit"] {
border: 1px solid #F3F3F3;
color: #333;
background-color: #F3F3F3;
background-image: -webkit-gradient(linear, 0% 0%, 0% 100%, from(#F3F3F3), to(#AAA));
background-image: -moz-linear-gradient(-90deg, #F3F3F3, #AAA);
background-image: linear-gradient(-90deg, #F3F3F3, #AAA);
-ms-filter: "progid:dXImageTransform.Microsoft.gradient(GradientType=0,startcolorstr='#f3f3f3', endcolorstr='#aaa')";
filter: progid:dXImageTransform.Microsoft.gradient(enabled='true',startcolorstr=#f3f3f3,endcolorstr=#aaa,GradientType=0);
}