How do I reset this back to the default white background with grew
gridlines? The problem only appears with this particular workbook.

I am not sure if this will work for you as I do not have a file to
compare it to for testing, but here is an excerpt from the help
files. I think this should help:

Reset workbook and worksheet settings
If you want to keep your Book.xlt or Sheet.xlt files, rename them and
save them in the Templates folder, usually located at:
C:\Documents and Settings\user_name\Application Data\Microsoft
\Templates

Do one or more of the following:
In Microsoft Windows Explorer, remove any template files from the
XLStart folder. The XLStart folder is usually located at:
C:\Program Files\Microsoft Office\Office11\XLStart

If the files aren't in the XLStart folder, look in the folder
specified in the At startup, open all files in box on the General tab
in the Options dialog box (Tools menu).
To restore original settings for workbooks, remove Book.xlt
To restore original settings for worksheets, remove Sheet.xlt

Hope that helped,

Mark