I am using SQL Developer 3.2.10.09.57 against Oracle 10 & 11 DBs. When exporting results to XLSX format using default CP1252 encoding) the process works as expected creating the XLSX which I open with Microsoft Excel 2010 (Version 14.0.6106.5005 - 32-bit). When I open the XLSX, I notice that the entire workbook is formatted in the default "Body Font" which, in my case, is Calibri 11 point; EXCEPT for any numeric value which is formatted in a different font, again in my case, Dialog 11 point. This formatting difference is also present in XLS exports (using the same Excel version). In previous versions of SQL Developer (3.1 and earlier), I did not encounter this formatting difference in the resultant XLS/XLSX object.
I have modified the encoding used with the same results and I have thoroughly reviewed the SQL Developer preferences without finding any setting that would attribute to this font difference. I was hoping someone would be able to point me to a known solution other than "just reformat it afterwards" which is my current workaround. In advance, thank you for taking a moment to consider this issue. Let me know if further information is necessary.
I'm no Excel expert, but you can read the results of my xls and xlsx exports there, where I note the same issue you mention even in 3.1. SQL Developer uses the java.awt.Font defaults of (DIALOG, PLAIN, 12) when it calls the API that writes out the Excel files. As far as I am aware, no request has been made, or work done, on adding a set of preferences for Excel export Font/Style/Size.
Here is one such request from the SQL Developer Exchange for "Various Components", but no explicit mention of Excel: