Users of Final Draft 12 / 11 can use Night Mode -- click HERE for more information.
Users of Final Draft 10 and older:
Go to Document > Page Layout > Colors. You'll see the selections for Text, Background and Invisibles colors. Click on the color swatch (Mac) or the down-arrow next to the swatch (Win) and a palette will open; select the color you want and click OK.
Note: This change of settings will affect only how the script looks on your screen; it will still print out in black lettering on a white page.