Ubuntu 16.04 - apt-get update - KEYEXPIRED
Solution 1:
I have the exact same issue (I use Arc theme), and this is more of a workaround. Technically this error is not harmful because it only deprives you of updates of the related software (presumably Arc Theme by Horst3180).
Go to Settings > Software & Updates > Other Software and uncheck http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 like so (see fifth row):
Solution 2:
Wait for the key owner to create/publish a new key. That's how it works, there is nothing to change, because nothing has changed.