Open up the terminal, and click on Terminal > Preferences... then Text and Colors. Change the background color to something dark, like black and your text to a somewhat light color, like white or light blue (you can tweak the text color later, also). Close that terminal window and open up a fresh one.
Type this in:
defaults write com.apple.Terminal TerminalOpaqueness 0.8
You can change that 0.9 to anything between 0.0 and 1.0. I find that anything between 0.7 and 0.9 is pretty good. Close the entire Terminal application and restart it. Voila!