EDIT: I also tried
@media only screen and (min-width: 1200px){
.blog .col-sm-4,
.archive .col-sm-4{
width: 25%;
}
.blog .col-sm-7,
.archive .col-sm-7{
width: 70%;
}
}
as written in https://colorlibsupport.com/t/how-do-make-my-blog-full-width/ without further success. Any help would be appreciated 