Dark theme is not good for your eyes.
When it comes to visual-acuity tasks and proofreading tasks, studies have shown that people with normal and corrected vision perform better with Light mode. This means that text in Light mode is clearer and quicker to understand for people with normal or corrected vision.
The researchers put this down to the fact that when we look at a bright background, our pupils constrict and increase acuity while scanning text. When looking at a black background, the opposite effect occurs, and dilated pupils make it harder to focus on the text. Dark theme also causes your eyes to dilate. Since there is less light to take in, your eyes have to work harder to see clearly. Eye dilation can reduce your vision's sharpness, so you may have to strain to see well.
I never use dark theme on anything.