Each of the controls included in the RadControls for ASP.NET AJAX suite is styled with two CSS files that are loaded in a certain order. The first one - ControlName.css (also called base stylesheet), contains CSS properties and values that are common for all skins, i.e it is layout-specific, not skin-specific. These are CSS float, padding, margin, font-size, font-family, etc. In the general case, when creating a custom skin for a control this file should not be edited, unless the custom skin needs different sizes, padding or margins.
The second file represents the actual skin of the control, and its name consists of the control name plus the skin name, i.e - RadSlider.Default.css. Upon creating a custom skin for the control, one should edit that particular file, as it contains skin-specific CSS properties, and references to images, colors, borders and backgrounds.
This an article from our help, that demonstrates how to create a custom skin: Tutorial: Creating RadSlider Custom Skin
You could also find useful the following help articles:
Understanding the Skin CSS file
CSS Skin File Selectors
If you have any troubles to customize your own skin, please send us a working project with custom sprite images and CSS file included, and will help you with implementing the customization.
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