The RadDockZone's placeholder is rendered with RadDock_Default rdPlaceHolder css class no matter of the skin which is applied, however, the RadDock_Default selector does not influence the appearance of the placeholder and should not confuse you. This setting does not lead to loading any additional axd files. The axd files which the RadDock controls loads when using a built-in skin are two - a base stylesheet file and a specific skin file. When you use a custom skin you need to set the EnableEmbeddedSkins property of the RadDock to false so that the second skin file will not be loaded. We recommend using the base stylesheet of the control even if you use a custom skin as it contains some common css settings of the dock, however, if you do not need it you could avoid loading it by setting the EnableEmbeddedBaseStylesheet property of the dock to false. I suggest you take a look at the Creating a custom skin help article for more details.
You mention that a bunch of axd files are loaded, we could recommend you check what kind of controls you have on the page and what are the skins they use. Most probably the axds files you see are loaded from other controls on the page.
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