This is a revision that addresses T92492: Spreadsheet does not change when the theme changes

Problem: The Blender Light, White, Print Friendly, and XSI themes weren't updated to color the Spreadsheet workspace appropriately.

Solution: I updated the themes so that the Spreadsheet would fit into the aesthetic of each theme.

note: I needed to make two separate diffs, one for the addons and one just for the Blender Light theme. I am submitting two revisions in total. This revision targets the Blender Light theme.

Blender Light before