Hey plateau!
Ok, I see the problem you are facing.
If you put a div box into the custom HTML section it does not get the responsiveness.
I would work with a little cheat.
I see that in the gridder you made a special box which will be styled with the class lay-hr.
I would do a second box, right above it and give it the class lay-hr-top.
Then use:
.lay-hr-top : {
margin-top: -200px;
border-top: 1px solid black;
}
I did not test the code, so maybe the 200px are too much. Maybe you also have to give the box a height.
Another solution would be to create a line in the custom HTML section, right beneath the body tag and use jQuery to always synchronise the width with the width of the gridder.
But I would go for the first option – even though it is not a very clean solution.
I hope that helped!
Best!
Marius