Ubuntu – Ubuntu 16.04 sometimes does not apply theme at startup


I have the arc theme installed and the numix circle icon pack. Sometimes, when I boot up the system, the desktop looks like this (note also how bad the text looks):

desktop at startup

However, if I do from the terminal sudo service lightdm restart, everything is normal as expected:

after restarting lightdm

This didn't happen some days ago, maybe it's something I installed. This also happened sporadically on 14.04, but now is more frequent.

How can I solve this, or at least how can I try to figure out what's wrong?