Please, find attached the default skin of RadSlider. You need to modify the vertical classes in order to achieve the desired result.
To use a custom skin, you have to set the EnableEmbeddedSkins="false"
property to the server tag of RadSlider, and then to register the two css files in the <head>...</head>
section of your webpage by using the <link>
You do not have to edit the Slider.css file, as it contains only layout-related properties, not skin-specific ones.
the Telerik team