Q: When adding a theme, why can't I add themes that are from a "Grid Data Source"?

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A:  The Spatial Analyst extension is probably not activated. To turn this extension on, go to the main menu and select File, then Extensions.  The Extensions dialog box will appear.   Select the Spatial Analyst extension by placing a check mark in the box beside the words Spatial Analyst and click ok.  Then, try adding the theme again as a "Grid Data Source".