Assume you want to do this to cell A1
Select cell A1
Goto the menu Format>Conditional Formatting
Select Formula Is
Enter this formula in the little box on the right:
=A1=""
Click the Format button
Select the style(s) desired. Maybe a fill color from the Patterns tab?
OK out
If you want A1 to be formatted when blank, then open conditional formatting
for A1, set the condition to "Formula is" and in the formula cell type
=ISBLANK(A1)
You can use the format painter to copy this to any other cell or range of
cells.