Changing Border color for Corporate Theme

This is not outer-border, its box shadow and you can change in style.css file (i.e.sites\all\themes\corporate\style.css) at line no:252.

.inwrap{
        background:#fff;
        -moz-box-shadow:0px 0px 5px rgba(0, 0, 0, 0.3);
        -webkit-box-shadow:0px 0px 5px rgba(0, 0, 0, 0.3);
         box-shadow:0px 0px 5px rgba(0, 0, 0, 0.3);
} 

To

.inwrap{
        background:#fff;
        -moz-box-shadow:0px 0px 5px rgba(0, 0, 0, 0);
        -webkit-box-shadow:0px 0px 5px rgba(0, 0, 0, 0);
        box-shadow:0px 0px 5px rgba(0, 0, 0, 0);
}        

Feel free to contact me incase of any concern/query regarding the same.

Thanks!