I would start by eliminating shadows which can be done by ensuring that every desk in the office is lit by at least two lights, ideally to the left and the right of the desk.

Also try to avoid reflections and glare by now having lights positioned directly above and in front of the desks.

In addition, use LED panels not fluorescent lights - they are sharper, and do not flicker.  

Ideally each office table should have a desk lamp so that the worker can improve their lighting above and beyond the overhead lights.

A desk light should be adjustable.

Sunlight is best - so if you have windows, you can position special reflectors on the outside of the windows that will let in more sunlight than usual (and conversely you can tint windows if too much direct sunlight is getting in).

The best thing you can do for your office is to install high grade LED panels - make sure they are high grade because the cheaper ones will yellow over time, whereas the better ones will not.

Office lighting should be cool white.

That said, you should have breakout areas where workers can take time off (or even have informal areas) and these should have warm white lighting to make things more comfortable.