Erebus is the god of Darkness. His darkness surrounded and filled the earth. Towards night time Nyx, his wife (also his sister) took his darkness and spread it across the sky bringing the night sky. At dawn his daughter, Hemera, cast his darkness away bringing daytime. His darkness was thought to be of the netherworld. The name Erebus is also used to describe hades’ realm.
paraphrased from http://www.theoi.com/Protogenos/Erebos.html
Erebus seems to be evil because his darkness was of the netherworld. "His dark mists encircled the world" (Theoi Project). From that I was able to gather that he was mysterious because he was made of shadow. Erebus was also strong and independent.
Inference from what I could gather from his myth
Greeks believed in a hell and a heaven. The gods were in the heavens and everyone else ended up among the fiery pits of hell. "Hell is a place of punishment after death or, in more abstract terms, a state of spiritual damnation" (Myths Encyclopedia). Erebus, being the god of shadows and darkness was thought to be of the Underworld.
no specific theme but what they valued and believed
Erebus is the oldest God
1). http://www.theoi.com/Protogenos/Erebos.html (AKA) Saunders, Chas, and Peter J. Allen, eds. "EREBUS - the Greek God of Darkness (Greek mythology)." <em>Godchecker</em>. Godchecker.com / CID, 23 Aug. 2015. Web. 8 December 2016.
2). https://en.wikipedia.org/wiki/Erebus (AKA) Wikipedia contributors. "Erebus." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 10 Nov. 2016. Web. 10 Nov. 2016.
3). "Myths Encyclopedia." Hell - Myth Encyclopedia - Mythology, Greek, God, Legend, Names, Ancient, Norse, Hindu, World, Chinese. JRank, n.d. Web. 13 Dec. 2016.