**Ring around the star.**

[![Ring around Fomalhaut][1]][1]

from https://commons.wikimedia.org/wiki/File:Fomalhaut_B_entire-Hubble_Telescope.jpg

Just as a planet can have a ring around it, so can a star.  If you were in the same orbital plane as the ring and it was between you and the star, it would shade you.  Consider the situation of the outer shepherd planet in this schematic of the ring around the star Fomalhaut.  It is shaded by the ring.

from http://www.solstation.com/stars/fomalhau.htm
[![schematic of Fomalhaut ring][2]][2]

This is nice for your scenario in that

 - You can have it suddenly get darker if you like.  The ring was formed by a dissolution of one of the inner planets in the system.

 - You have no constraints on the atmosphere of your own planet: the shade is cast by far away stuff.  Degree of shade might vary with the density of the interposed ring, which can change as ring and planet orbit.

 - What would this look like from the perspective of the planet?  I am sure there would be twinkling chunks and haze in the sky at all time.

 - You planet can have one pole protruding beyond the shade of the ring.  Here one can still see the sun rise.


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ADDENDUM - Ring are thin, I hear?  Too thin to shade?  Saturns rings are thin compared to Saturn or compared to their width, but we are talking about a ring around a star.  How thick is Fomalhaut's ring?

http://www.dailygalaxy.com/my_weblog/2012/10/the-strange-planets-of-fomalhaut-a-spectacular-alien-star-system.html

> The original ALMA research shows that the ring's width is about 16
> times the distance from the Sun to the Earth, and is only one-seventh
> as thick as it is wide.

So 16 / 7 = 2.2: the ring is only as thick as double the distance from the Earth to the Sun, which is 146 million km x 2 or 292 million km.  The diameter of the Earth is 12,742km.  I think the Earth would be able to find shade from a ring of this size.
 
  [1]: https://i.sstatic.net/H3kOs.jpg
  [2]: https://i.sstatic.net/E2w4m.jpg