Change Mobile toggle menu to move from bottom

Hello,

I’m using a bottom sticky nav and would like the toggle to open the menu above the bottom (so dropping upwards instead of dropping down). A nice example is the mobile nav on this: http://meldebarge.com.

I’ve gotten it to work ok using a negative margin like this the code below. However, if I make the background of the div a solid color so that it covers up the background of the page I can’t get the background to span the full width of the page. I’ve tried adding padding but it doesn’t work right.

I suspect this isn’t the best approach to begin with – if anyone can make a suggestion or point me in a good direction I’d really appreciate it. thanks!

.menu-mainnav-container {

margin-top:-200px;
background:#FFF;
width:100%;

}