Basically StyleSheetTheme is a Theme which gets applied in very early page cycle before the page control properties applied.

So order is like
    StyleSheetTheme -> Page -> Theme

That means control property set in StyleSheetTheme can be overridden by the control property in Page and control property set in Page can be overridden by the control property in Theme.

For example if StyleSheetTheme contain following default Label skin.
<asp:Label runat="server" Text="StyleSheetLabel" Font-Size="Small" BackColor="Red" ></asp:Label>

• StyleSheetTheme works in Designer
• To apply StyleSheetTheme at Site Level define following in web.config
  

  <system.web>
        <pages styleSheetTheme="Theme1" />
  <system.web>

• To apply StyleSheetTheme at Page Level

  <%@ Page Language="C#" StylesheetTheme="Theme1"%>

• To apply StyleSheetTheme dynamically user need to override the StyleSheetTheme property.
  

  public override string StyleSheetTheme
  {
     get{ retrun "MyStyleSheetTheme"; }
  }

• To apply a StyleSheetTheme to a control added dynemically user need to call ApplyStyleSheetTheme(Page) method on the control.
  
  Label lab1 = new Label();
  lab1.ApplyStyleSheetSkin(this);

• EnableThemeing doesnt work with StyleSheetTheme. Setting EnableTheming="false" doesnt stop applying StyelSheetTheme to the page or control.

• Can not apply SkinId dynemically.