If the shading you want to change really is table cell shading, then (a) it is not .Font.Shading and (b) you can't use Range.Find to locate it. [There are three possible kinds of shading in a table cell, controlled by the dropdown in the Shading page of the Borders & Shading dialog: Text, Cell, and Table. In VBA each of them is addressed differently.]
This macro iterates through all the cells of each row of each table in the document and makes the change when it finds the proper color of Cell background shading.
Code:
Sub test()
Dim tbl As Table
Dim rw As Row
Dim cel As Cell
For Each tbl In ActiveDocument.Tables
For Each rw In tbl.Rows
For Each cel In rw.Cells
If cel.Shading.BackgroundPatternColor = RGB(225, 225, 153) Then
cel.Shading.BackgroundPatternColor = RGB(225, 225, 225)
End If
Next cel
Next rw
Next tbl
End Sub