Excel has always been a great program for analyzing numerical data, but even Excel 2003 came up a bit short in the presentation department. In Excel 2003 and earlier versions of the program, you could have a maximum of 56 different colors in your workbook. In addition, there was no easy way to ensure that your colors complemented the other colors in your workbook (unless you were a graphic designer and knew what you wanted going in).
Excel 2007 offers vast improvements over the color management and formatting options found in previous versions of the program. You can have as many different colors in a workbook as you like, for example, and you can assign a design theme to a workbook. Assigning a theme to a workbook offers you color choices that are part of a complementary whole, not just a dialog box with no guidance about which colors to choose. You can, of course, still select any color you want when you format your worksheet, define custom cell styles, and create your own themes. The preinstalled themes are there as guides, not prescriptions.