SAP GUI theme: In an InputScript I would like to ascertain the SAP GUI theme that the user has selected in the SAP GUI options, e.g. Signature or Blur Crystal theme. Is there a GuiXT system variable that I can use?
There is no system variable available. You can read a value in the registry to detect the active theme, e.g with the CallVB statement (GuiXT Controls component). The SAP GUI theme is stored in

[HKEY_CURRENT_USER\Software\SAP\General\Appearance]

"SelectedTheme"=dword:00000080      (128=Blue Crystal Theme)

The value for the themes are:

Signature 1
Enjoy  2
System specifc 4
Corbu: 64
Blue Crystal Theme: 128