Are you editing the .css files? If so then that's your problem - you need to edit the LESS files:
Ohhh, I have read several times that it´s made with less... Thanks.
Actually I found a bug just now and fixed it, https://github.com/NodeBB/NodeBB/commit/5ff293109d90dae5345b3afa7fbe8679550485d3 it should be good from now on.
barisusakli committed to NodeBB/NodeBB