Need some help getting an element to have a fixed bottom position

[vinnybear]

On a site (genesisstudios.net) I have developed a different kind of mobile menu. The button is at the bottom, and that part is working fine.
The menu that pops up from the bottom after tapping the menu button seems impossible to make the position fixed to the bottom.
Currently I have some CSS to try and make it do this, but it is just a workaround.

#et-boc .area-outer-wrap[data-da-positionv=center]{
top: 64% !important;
}

Any help would be much appreciated!
FYI, you can only view the menu in mobile screen size.
Thanks!