Altering the way WP builds emnu

OK, so here’s the thing.

I’m creating my first WP theme. I used Thematic as a base and my theme is it’s child.

Now, what I want to have on my site are two menus (just for the record.) The main menu is in top right and I’ve styled it already. The other menu is supposed to be somewhere near the center (I’ve this styled too.) The trouble is: I want the menu to be tiles, like those from W8 but all of them will be of the same size. Because of that I needed it to be in the center, so I made something like that:

li {
display: table;
}
a {
display: table-cell;
vertical-align: middle;
}

But then there comes the third thing I want and that’s creating a fluid layout. I’ve got it all done (almost) ’cause the last thing I need is making those two fluid.

In order to achieve that I want each of them to be 33% of the width and I to be squares and I want them to be fluid, too. I found this article which explains how to do that: https://ansciath.tumblr.com/post/7347495869/css-aspect-ratio. The only trouble is, this calls for altering the way WP builds menus (I need to add one or two elements to it) and I can’t really find how to do that.

Thanks in advance for your help.
k