As I have seen on your website you have multiple navigation levels and I didn't know which level, so I'll give you the two examples.
If you mean the first level you can add following to your css file:
.children li {
background-color: beige;
}
If you mean the level underneath the children, do following:
.children li ul li {
background-color: beige;
}
I you would like to change the text color do like this:
first level
.widget .children li a {
color: #1fb4b9;
}
second level
.widget .children li ul li a {
color: #1fb4b9;
}