In your ObjectDock installation folder, i.e. C:\Program Files\Stardock\ObjectDock\ two png images must reside called minus.png and plus.png. If they are there, they should show automatically.
You can define them in a theme ini file though, by adding these lines:
[Extras]
PlusImage = Backgrounds/6070b Tabs/6070b_plus.png
MinusImage = Backgrounds/6070b Tabs/6070b_minus.png