Dashboard background cell colouring removed by pivot when resized smaller

I have a dashboard with graphs (January to December) and the supporting Pivot data displayed below (with Months reading across in columns left to right) - 13 columns in all (Field Title + 12 months). Says Cells A1:M2.
I have coloured grey the cells of the dashboard not taken up by Pivots/graphs.

However, when I dynamically change the Pivots to just display less than 12 months of data ...and the Pivot compacts the number of columns (eg for just January, the pivot only needs Cells A1:B2)- the column cells vacated by the Pivot (Cells C1:M2) just go white. The old grey dashboard colouring is removed.

How can I keep this, regardless of how the pivot resizes itself according to the scope of data selected in timeline/filters?

Thanks.