In the upper-right corner of the format window is a little drop-down menu labeled Fields. Selecting this drop-down menu gives you a list of fields in the current view, and selecting a field updates the format window with options appropriate for the field. Here, for example, is the window as it appears for the SUM(Sales) field:

The title of the format window will alert you to the field you are formatting. Selecting an icon for Font, Alignment, and so on will switch back to the sheet-level formatting. However, you can switch between the tabs of Axis/Header and Pane. The options for fields include Font, Alignment, Shading, and Numbers and Date formats. The last two options will override any default metadata formats.