Beware! If you are getting eyestrain from a light background, it is almost certainly due to working in a dark room. While light vs. dark is usually a moot question if the background matches the room lighting, a bright background reduces the viewer’s pupil and therefore increases depth of field. This makes focusing on the content much easier for people with mild to moderate vision impairment. Sadly, that’s many of us, especially us geezers dealing with presbyopia. You could say that dark themes are ageist, and should not be used where you have a wide variety of users.
Some reading on the topic: