Dynamic themes: Themes which change color from time to time, based on their local machine time.
(Obviously, these color theme style should be good for eyes, something similar to white color on day light and black color during night-time, It would be even better to change color slowly such that user doesn't notice it, transition time is high around 12 hours)

Officially integrate redshift or something similar by default or at-least an option, rather than widget which needs to be installed by user. (It really helps, some users may not known about them, and if installed by default they will probably like it, It helps every one and feels good on eyes)

If Dynamic themes have several performance issues, then some kind of option to change theme within time range.
Ex:-

Breeze theme during day-light(from 6:30AM to 7:30PM) and then,
Breeze Dark theme during night-time(from 7:30PM to 6:30AM).

Thank You