I've got the same problem. It is not the best solution, but until this solution comes... it could help you. Try adding this to your CSS file with your own colours and measures:
.gsc-search-button
{
background-color: #1a4195;
border-radius: 5px;
}
input.gsc-search-button, input.gsc-search-button:hover, input.gsc-search-button:focus {
background-color: #1a4195 !important;
background-image: url("http://www.google.com/uds/css/v2/search_box_icon.png") !important;
background-position: 50% 50%;
background-repeat: no-repeat;
height: 16px !important;
border-color: #1a4195 !important;
filter: none;
}