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The control has two options, tree and dropdown list. To make the control behave like a drop down list, click edit on the archive control. Then, go to Behavior > ArchiveMode and change the property to drop down list.
To modify the CSS for the control in RadTreeView mode, you must create a custom skin
. I have attached a user control, which you can adjust according to those custom skin instructions. Please copy the control to ~/Sitefinity/ControlTemplates/Blogs folder. Please set the LayoutTemplatePath property of the Archive control, which is located in the Appearance section of the control, to the location of the archive template. You can use the CSS class of the drop down list control to alter appearance, when the control is in drop down list mode.
I hope that I have clearly answered your technical questions. Kindly let us know if you need further assistance.
the Telerik team