This is a 4-colour no-red cube.
The themes are centered around growth and decay, birth and death:

• +1/+1 counters (support, outlast, proliferate)
• life gain (triggers)
• ETB triggers & bounce
• Death triggers
• -1/-1 counters (infect, wither, proliferate)
• poison (infect, toxic, proliferate)
• graveyard (sacrifice, delve, reanimating)

Green wants to focus on almost all of the themes: toxic, infect, counters, life gain, proliferation.

White wants to focus upon toxic tokens, counters, life gain, ETB, bounce.

Blue supports control decks, brings a lot of proliferation, ETB, bounce.

Black wants to pair with white to trigger life loss from life gain, or pair with green to use sacrifice, reanimate, or focus upon a -1/-1 and/or poison strategy.