Changing the default vim theme is quite simple yet so many don't bother to spend two minutes to improve their quality of life dramatically.
Here's how to do it:
Download the 256-grayvim.vim colour scheme or any other scheme you like. Make sure that the 256-grayvim.vim file ends up in the following directory (you'll need to be root):
/usr/share/vim/vimXX/colors/ (replace XX with the installed vim version, e.g. 71, 73, or 74)
Then add the colour scheme as default for all users:
Add the following to the very bottom of the file:
Save and exit. Next time you open any file with vim the colour scheme should be applied.