You can copy and paste chart formats. In 2003 and previous versions you copy the graph and then use
Edit>Paste Special to paste only the formats. I've no idea of the equivalent ribbon control in 2007/2010 I'm afraid, but I'm reasonably sure it should be there (MS did remove some charting functionality in 2007).
However, with 900 charts even this may a bit of a pain. You could use a bit of VBA to achieve the same though.
Edit: Had a bit of a play about and this will copy the format of Chart1 to all other charts in the worksheet:
Dim c As ChartObject
For Each c In ActiveSheet.ChartObjects
Again, be aware that this has only been tested in Excel 2003 and the object model in 2007 is missing a few things compared to earlier versions, but this is pretty fundamental stuff so should work.