Format>conditional formatting>formula is

=A2=""

click format button and pattern, select yellow and click OK twice

replace A2 with the first cell of a selected range for instance if you want
this to
happen in C2:C100, select that range and use

=C2=""

with C2 as the active cell

Select the cells to format and click FORMAT in the menu and select
CONDITIONAL FORMATTING.

In the CONDITIONAL FORMATTING dialog box, change CONDITION 1 to FORMULA IS
and in the text box to the right enter the following formula, changing the A1
in the example to the starting cell of your selected range:

=ISBLANK(A1)

Or if you want to treat any errant spacebar entry as a blank cell

=LEN(TRIM(A1))=0

Click the FORMAT command button, select the PATTERNS tab and select the
color you want and click OK 2 times to exit.