Twitter Bootstrap + less + Responisble design - Add span4 to my class

[Tomas Randus]

Hi there, please help me:

I would like to prepare own responsible design for my HTML 5 page.

If I define something like:

#header {
    .span4;
}

it throws less compile error seems to be kind of the .span4 not found.

I would like to change the column count of elements for each display
size.

Thank you for any help.