code: function initControls() {
      if (!nextBtn) {
        // create & append next-btn
        nextBtn = document.createElement('span');
        addClass(nextBtn, _const_class_prefix + '-next');
        
        // add custom images
        if (CTX.opt.nextImg) {
          var nextBtnImg = document.createElement('img');
          nextBtnImg.setAttribute('src', CTX.opt.nextImg);
          nextBtn.appendChild(nextBtnImg);
        }
        else {
          addClass(nextBtn, _const_class_prefix + '-no-img');
        }
        addEvent(nextBtn, 'click', function (e) {
          stopPropagation(e); // prevent closing of lightbox
          CTX.next();
        }, false);
        CTX.box.appendChild(nextBtn);
      }
      addClass(nextBtn, _const_class_prefix + '-active');
      if (!prevBtn) {
        // create & append next-btn
        prevBtn = document.createElement('span');
        addClass(prevBtn, _const_class_prefix + '-prev');
        
        // add custom images
        if (CTX.opt.prevImg) {
          var prevBtnImg = document.createElement('img');
          prevBtnImg.setAttribute('src', CTX.opt.prevImg);
          prevBtn.appendChild(prevBtnImg);
        }
        else {
          addClass(prevBtn, _const_class_prefix + '-no-img');
        }
        addEvent(prevBtn, 'click', function (e) {
          stopPropagation(e); // prevent closing of lightbox
          CTX.prev();
        }, false);
        CTX.box.appendChild(prevBtn);
      }
      addClass(prevBtn, _const_class_prefix + '-active');
    }
docs: Initializes the control arrows