ExcelXP does NOT have a particular style definition that it works with, not like RTF or PDF. If there were ANY style definition that I would start with for ExcelXP, I would look at STYLES.DEFAULT as the style template to start with.
However, from your question, it seems that you are interested in knowing which style ELEMENTS and style ATTRIBUTES you can change. In this example of proc template code:
style data /
DATA is a style ELEMENT
FOREGROUND and BACKGROUND are style ATTRIBUTES
But, it doesn't exactly matter what your destination is. What SAS is creating are primarily TABLES from SAS procedures and tables themselves are pretty simple.
The column headers are controlled by the HEADER style element; the data cells by the DATA style element (or sometimes by the CELL style element) and the TABLE itself (interior table lines, borders etc) is controlled by the TABLE element (or sometimes by the OUTPUT element). The background that the table is sitting on is generally controlled by the BODY element.
What I have found when working with the ExcelXP tagset is that most of the straightforward elements -- HEADER, DATA, TABLE and the straightforward attributes -- BACKGROUND, FOREGROUND, RULES are respected by Excel when you use a particular style template change with the ExcelXP tagset.
When you change a template and use it with ExcelXP -- I recommend making one change at a time -- so you know what is working and what isn't working. The Tech Support FAQ on PROC TEMPLATE will be of great help to you (listed in the previous postings). If you have specific style element and ExcelXP behavior issues, your best bet for a quick response may be to contact Tech Support for further help.