Which LESS file do I add Media Queries on the T3 blank template (Bootstrap 2)

I am working in the development mode, and I feel that I am adding Media Queries in the wrong place because they are not working correctly.

I want to add a margin of 40px to the sidebar modules if the screen size is less than 1824px.

So my code is :

@media (max-width : 1824px) {
.t3-sidebar {margin-top: 40px;}
}

But this seems to add the margin to all screen sizes.