Glenda, yep, a thin gray outline is absolutely do-able. I even played with one briefly but left it off for the following reasons:
1. I was trying to mimic the current look first,
2. I felt the empty cells were mostly noise so letting them recede was better,
3. I thought it looked crappy (my opinion) and was very busy.
They could just as easily be added with another toggle checkbox. Maybe when I get back from Fat Tuesday gluttony I will add one.