What is a StyleSheetTheme?
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>
and Page Which has StyleSheetTheme and Theme defiened contain a Label contol as
<asp:Label runat="server" Text="PageLabel" Font-Size="X-Large" ></asp:Label>
and Theme contain following default label skin.
<asp:Label runat="server" Text="ThemedLabel" ></asp:Label>
Then the resultant Label shown will look like.
<asp:Label runat="server" Text="ThemedLabel" Font-Size="X-Large" BackColor="Red" ></asp:Label>
Above example is just to show the precedence heirarchy but generally either StyleSheetTheme or Theme will be applied to the page and not both.
StyleSheetTheme has same characteristics Theme except few differences.