Hi,
To change the white color, you can add the code below in Theme Options > CSS
/* Entry Navigation background color */
.x-nav-articles a {
background-color:red;
}
.entry-top-navigation .entry-parent {
background-color:red;
}
/* Entry Navigation hover color */
.x-nav-articles a:hover {
background-color:blue;
}
.entry-top-navigation .entry-parent:hover {
background-color:blue;
}
Change red and blue with the color that you like.
Hope that helps