To control dynamic devices with a slider (i.e. dimmers), you can add a slider element to the remote. Just click on the slider symbol to add a slider.
You can assign an action and/ or a status device to the slider in the properties menu. Optionally you can set checks to turn the slider vertical (normal or inverted) and show the value. You can also change the size and the style of a slider.
To select different styles for a slider, click on the style-button in the properties menu. Now select the tab “image based” to select a style which is based on images. You can not modify these image based slider styles in the NEO user interface.
If you select the “CSS” tab, you can modify the slider by CSS commands. → more about CSS-Slider