The problem
My monitor is a Dell S2716DG, a TN panel which is notorious for its posterization artifacts when viewing dark content. Its color performance is poor, particularly around the extreme low end of the color gamut. Each pixel has three color values, one for each red, green, and blue, which (being an 8-bit color depth per component display) range from 0-255. Across this range, each increment should be small enough to be indistinguishable, otherwise the display would not be able to produce smooth gradients. On the S2716DG, this is mostly true about the upper end of the 0-255 range, but from (0,0,0) to (1,1,1), (1,1,1) to (2,2,2), ...all the way up to about (10,10,10), each increment is seemingly the relative luminance between a keychain light and the sun. So much of the monitor's contrast ratio is expended in this range that in order to get dark games like Thief to look right (i.e. not exceptionally bright) the gamma has to be cranked nearly all the way down. The problem then becomes the ubiquitous posterization artifacts which litter the image nearly 100% of the time.

The solution
Enter dithering. Dithering is a way to create the illusion of increased color depth by introducing a bit of strategic noise into the image. This is the effect I need to combat posterization since it will create the illusion that my monitor is capable of reproducing many more color values within that crucial (0,0,0) to (10,10,10) range. Fortunately, NewDark renders at a much higher color depth than what gets displayed and it lets users roll their own postprocessing shader to control how that high color depth render target gets crunched down to the displayable format. Here, I've written a shader (based on ones included with NewDark) which enables randomized dither to eliminate the posterization.
1. Stick the shaders folder in the same directory as Thief/Thief2.exe
2. Set d3d_disp_sw_cc 2 in cam_ext.cfg

Some comparison shots:
Example 1. Dithering off | Dithering on
Example 2. Dithering off | Dithering on
Example 3. Dithering off | Dithering on

If for you these screenshots are so dark as to be illegible, congratulations, you have a reasonable monitor and probably won't benefit from using this shader. On my monitor these shots are completely 100% legible under the normal low ambient light conditions in which I play Thief. If you can open these comparison shots and clearly see the posterization artifacts, you will probably benefit from using this shader.