This is a dark, semi-transparent theme with a Skull and Crossbones. The Skull and Crossbones disappear when the calculator is in use.
This is for use with Screenlets. To install open the Screenlets Manager, click on Calc the on the left click on Install New Theme. Restart the Calculator and then new theme will be available.
Edit: My thanks to Claudiam. This theme is an adaptation of the one he created.