function change_active (element) {
if(element=="oferty_pracy") {
    document.getElementById("oferty_pracy").className="middle-bottom-title-active";
    document.getElementById("oferty_praktyk").className="middle-bottom-title";
    document.getElementById("oferty_trenerzy").className="middle-bottom-title";
    }
if(element=="oferty_praktyk") {
    document.getElementById("oferty_pracy").className="middle-bottom-title";
    document.getElementById("oferty_praktyk").className="middle-bottom-title-active";
    document.getElementById("oferty_trenerzy").className="middle-bottom-title";
    }
if(element=="oferty_trenerzy") {
    document.getElementById("oferty_pracy").className="middle-bottom-title";
    document.getElementById("oferty_praktyk").className="middle-bottom-title";
    document.getElementById("oferty_trenerzy").className="middle-bottom-title-active";
}
return true;
}