$(window).scroll(function() {
var top = $(window).scrollTop();
if(top>0){
$('.header').addClass('header--min');
} else {
$('.header').removeClass('header--min');
}
});
.header--min{}
задавай изменения состояния. "devDependencies": {
"browser-sync": "^2.18.8",
"gulp": "^3.9.1",
"gulp-autoprefixer": "^3.1.1",
"gulp-concat": "^2.6.1",
"gulp-csscomb": "^3.0.8",
"gulp-csso": "^2.0.0",
"gulp-newer": "^1.3.0",
"gulp-plumber": "^1.1.0",
"gulp-rigger": "^0.5.8",
"gulp-sass": "^3.1.0",
"gulp-tinypng": "^1.0.2",
"gulp-uglify": "^2.0.1",
"gulp-watch": "^4.3.11"
}