Whilst I love Flash Builder, it is somewhat daunting navigating the large amount of preferences. So, if you wish to change the font, size, and colour of your editing window here is where you go…
Window > Preferences > General > Appearance > Colours & Fonts > Basic > Text Font
For the background colour and stuff like that you go to …
Window > Preferences > General > (and click on the text) Text Editors
At the bottom of the panel you will now Foreground colour, Background colour etc.
