title: vim in screen: vim does not highlight syntax when I'm in a screen session
body: <p>I cannot see any syntax highlighting in any language (e.g. python, c++ and sh) when I use vim within a screen session. The line numbers are in color though. 
  I precise that my terminal (in screen too) is able to show 256 colors schemes (I tested with the 256colors perl script found here: <a href="http://frexx.de/xterm-256-notes/" rel="nofollow">http://frexx.de/xterm-256-notes/</a> ).</p>
  
  <p>How can I fix that?</p>