Dark to light is a thought-terminating cliché uttered by believers in the QAnon cult. They say it when they are excited, or just for no reason, as a signature of sorts. While QAnon cultists claim to think for themselves, "dark to light", like most QAnon thought-terminating clichés, was first uttered by the Q poster himself.
It is a corruption of the words of Jesus in John 8:12:
Then spake Jesus again unto them, saying, I am the light of the world: he that followeth me shall not walk in darkness, but shall have the light of life.
However, QAnon cultists are not good Christians, as they believe that QAnon will "save the world" via Trump, and not Jesus Christ, in direct repudiation of the Bible they claim to believe in.
Of course, it means that QAnon can do no wrong, as anything he does that appears wrong is necessary to "bring about dark to light". This is clearly a cult indoctrination tactic to make his believers follow him unquestioningly.
- The Facts: What Christians Should Know About QAnon, The Gospel Coalition. [The FAQs: What Christians Should Know About QAnon]