The controls' and skin-specific styles are located in the C:\Program Files (x86)\Telerik\UI for ASP.NET AJAX [your version]\Skins
As written in the CSS Skin File Selector
article you could use the .RadDropDownList_Sunset
class to set the yellow color to all the DropDownLists with Sunset skin.
I would like to focus your attention on our new Theme Builder
which will help you create easily custom skins by modifying existing ones.
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and share your thoughts.