Change background color of entire row based on value in a cell
Use "custom formulas".
Let's say you have 25 rows with data validation, then go to the first row cell and right-click then select "conditional formatting". Put this as the rule....
Custom Formula =$A$1:$A$25="Completed" background color - dark gray range: 1:25
Now when you choose "Completed" in a row the whole row changes to a dark gray. Just change the range numbers in the formula to the number of actual rows that has data validation in them. Like if you have a 100 rows, then the rule will look like this...
Custom Formula =$A$1:$A$100="Completed" background color - dark gray range: 1:100