My version of SQL Developer (220.127.116.11) has white text on a light blue background (see below). I have been unable to find in the preferences to change this so that it is more readable. Does anybody known how to make the text a difference colour.
SQL Developer 3.x had a "theme" option, but that's been removed in 4.0, so no luck there.
You could try changing the "Look and Feel" option to Windows (I don't know what you get on other OSes).
Result should match your OS color scheme. I get this for example, which is a bit more readable than the default white on blue: