You can do it for the rear side windows only at whatever percentage you want.

For front and rear window as well as front side windows, it is not allowed, unless it is already slightly shaded by the manufacturer.

If the police find out you have done it as an addition (even 5%) you will be fined!

You have to have it permitted in your car's registration card. It costs 20 QAR to add, if you have already issued the card without this note...