It's very frustrating. You can increase the intensity of an image in Word (right-clicking "Format Picture"), but I don't know of a way to make text darker.
Here's my somewhat imperfect workaround:
I print out a document, then carefully put the sheet back in the printer and print a 2nd layer of type atop the first. Sometimes it works perfectly & makes a big difference, and other times the registration is way off. TIP: Make the page margins very small, and put the type you're going to layer right at the top of the page, giving it less chance to go askew.