title: How to remove white border in gVim 8 with gtk 3
body: <p>I recently updated my gVim to version 8 compiled with gtk 3, and I found there are strange inner borders around the gVim window:</p>
  
  <p><a href="https://i.stack.imgur.com/HEESb.png"><img src="https://i.stack.imgur.com/HEESb.png" alt="gVim 8 white borders"></a></p>
  
  <p>This picture may not very clear but if you look carefully you can see the white borders on the right side and the bottom. I have solved this issue by setting a dark color to the border on gtk 2, but on gtk3 it happends again. Any idea of removing this border completely on gtk 3?</p>