The domain establishes the sacred theme.
How to interpret and apply a theme depends on the community.
The Cleric with a sacred community, is much like a Warlock with a pact: the player needs to decide what the concept is, and what expectations for the relationship will be, and the DM needs to figure out how to make that work within the setting.
Even a sun being can associate with a Darkness Domain. For example, an eclipse concept. Especially, the concept might be that the sun disk sets and sinks below the horizon to then travel underground thru the realm of the dead to then rise back up in the morning on the other side. This darkness-of-night during a journey-of-death can be a meaningful sacred concept.
It depends on what the player and the DM figure out together. The sacred community and its traditions are key to making a Cleric concept work.
Again a "sacred community" can be devoted to any cosmic principle or ideal.
I am unaware of this. What are you referring to? In 5e?