Hi Pietro,
For T3 Blank, it's a little bit different, Here is the how-to, open the "variables.less" file in "templates/t3_blank/less", find the grid declaration:
// GRID
// --------------------------------------------------
// Default 940px grid
// -------------------------
@T3gridWidth: 940px; // T3 add. For non-responsive layout.
@gridColumns: 12;
@gridGutterWidth: 40px;
@gridColumnWidth: floor((@T3gridWidth - @gridGutterWidth * (@gridColumns - 1)) / @gridColumns);
@gridRowWidth: (@gridColumns * @gridColumnWidth) + (@gridGutterWidth * (@gridColumns - 1));
// 1200px min
@T3gridWidth1200: 1200px; // T3 add
@gridGutterWidth1200: 40px;
@gridColumnWidth1200: floor((@T3gridWidth1200 - @gridGutterWidth1200 * (@gridColumns - 1)) / @gridColumns);
@gridRowWidth1200: (@gridColumns * @gridColumnWidth1200) + (@gridGutterWidth1200 * (@gridColumns - 1));
// 980px-1199px
@T3gridWidth980: 940px; // T3 add
@gridGutterWidth980: 40px;
@gridColumnWidth980: floor((@T3gridWidth980 - @gridGutterWidth980 * (@gridColumns - 1)) / @gridColumns);
@gridRowWidth980: (@gridColumns * @gridColumnWidth980) + (@gridGutterWidth980 * (@gridColumns - 1));
// T3 Add: 768px-979px
@T3gridWidth768: 740px; // T3 add
@gridGutterWidth768: 20px;
@gridColumnWidth768: floor((@T3gridWidth768 - @gridGutterWidth768 * (@gridColumns - 1)) / @gridColumns);
@gridRowWidth768: (@gridColumns * @gridColumnWidth768) + (@gridGutterWidth768 * (@gridColumns - 1));
Now change the value of @T3gridWidth of responsive layout you want.
IMPORTANT: after changing, please compile LESS to CSS so that when you disable dev mode, the changes will be applied.
Gardner.