There are three primary motifs in The Oresteia: 1) Light 2) Dark and 3) Nets. (A "motif" is an object or idea repeatedly used in literary work.)
The use of light and darkness is a literary trope (a word, phrase, or image used in a new and different way in order to create an artistic effect) and it is evident in this very early play (which was first performed in 458 BC in Athens, Greece at the Festival of Dionysia).
As the play begins, society is in darkness. As it moves through time, and society becomes more civilized, i.e., "enlightened," things become more illuminated.
(NOTE: Line numbers rather than page numbers have been provided here for you. The source used appears below this answer).
The word "dark" occurs at least twelve times in the play. The Chorus references this darkness many times:
But once a murdered man’s dark blood 1200
has soaked the ground, who then
can bring him back through song?
Even Aesculapius, whose skill
could raise men from the dead,
was stopped by Zeus’ thunderbolt.
Was that not warning to us all?
If one fate settled by the gods
did not prevent another fate
securing an advantage,
my heart would then outrace my tongue— 1210
I’d speak out loud and clear,
I’d cry out my forebodings.
But now it mutters in the dark,
uneasy, holding little hope
for any resolution.
Importantly, the Furies represent darkness. They are dressed completely in black; the surround the home. They cause people to go mad, a type of symbolic darkness. Until the Furies leave, darkness will dominate.
The word "light" occurs even more often, a total of twenty-three times. Here, the Herald tells the Chorus:
If some ray of sunlight finds him still alive, 811
his vision still intact, thanks to Zeus,
whose crafty plans at this point don’t include
destruction of the entire race, there’s hope
he’ll soon come home again. Now you’ve heard this,
you’ve listened to the truth.
And here, the Herald, referring to Agamemnon, tells the Chorus:
He’s coming here, 623
carrying light into this darkness, for you
and all assembled here—our mighty king,
lord Agamemnon. Greet him with full respect.
For he’s uprooted Troy—with the pick axe
of avenging Zeus he’s reduced her soil.
The altars of the gods and all their shrines
he has obliterated, laying waste
all that country’s rich fertility.
References to "nets" occur ten times during the play. Nets represent entanglement, entrapment, and betrayal.
Agamemnon: Around Troy we’ve cast a savage net. (*Entrapment)
Clytemnestra: If my husband
had had as many wounds as I heard rumours
coming to this house, he’d have more holes in him
than any net. (Lines 1020-123) (*Betrayal)
Objects, too, not just ideas, appear in the net motif, especially Agamemnon's robe, upon which Cassandra exclaims (referring to Clytemnestra):
She’s caught him in her robes— 1330
now she gores him with her black horn.