В extra.less добавляем:
.linear-gradient-prefixes(@deg, @start, @end) {
background-image: -webkit-linear-gradient(@deg, @start, @end);
background-image: -moz-linear-gradient(@deg, @start, @end);
background-image: linear-gradient(90deg, @start 0%, @end 100%);
}
.linear-gradient(@_...