Vim

Dark background:

:set background=dark

Block selection

Comment

1. Place cursor on the first line you want to comment,
2. Press Ctrl+V to put the editor in the VISUAL BLOCK mode,
3. Select until the last line using the arrow keys,
4. Press Shift+I, to put the editor in INSERT mode,
5. Press # to insert a hash to the first line,
6. Press Esc (give it a second), and VIM will insert a # character on all other selected lines,

   For the stripped-down version of vim shipped with debian/ubuntu by default, type : s/^/# in the forth step instead.

Uncomment

1. Put your cursor on the first # character,
2. Press Ctrl+V (or Ctrl+Q for gVim),
3. Go down until the last commented line,
4. Press x, to delete all the # characters vertically.