There’s a couple places in the theme’s style.css file, the first being around line 124:
#content-wrapper {
min-height:200px;
background-color:#f8f8f8;
border-bottom:5px solid #595a67;
border-radius:60px 60px 0 0;
-webkit-border-radius: 60px 60px 0px 0px;
The second around line 153:
/* Secondary wrappers */
#nav-wrapper {
background-color:#78a5b6;
border-radius:60px 60px 0 0;
-webkit-border-radius: 60px 60px 0px 0px;