In a new Mozilla browser, click "Edit" then at the bottom of the drop-down list, click "Preferences..." That should open up a new window with some info on the right and a list of items off to the left. The very top item on the left should be "Appearance." Click the plus sign beside it to expand it, then select "Colors." There should be a "Text and Background" section which shows the default text and background of your windows. Set those to whatever colors you like and click "OK" to apply them.
Also, be sure to look at the radio buttons just below the Text and Background settings. One should say "Always use the colors and background specified by the web page" and the other should say "Use my chosen colors, ignoring the colors and background image specified." If you don't want to have the same background color for every page, select the first option and the browser will set the background and text colors depending on the website you're accessing, otherwise choose the second one if you want blue or green or any other color to always be your background/font color for the browser.
That's how it works with Mozilla v1.3, anyhow (I should probably update my version... lol). Hope that helps.