The Modus themes can be conceptually simplified as combinations of color values that account for relative luminance and inner harmony. Those qualities do not guarantee that every end-user will have the same experience, due to differences between people, but also because of variances in hardware capabilities and configurations. For the purposes of this document, we may only provide suggestions pertaining to the latter case.
modus-operandi is best used outdoors or in a room that either gets
direct sunlight or has plenty of light. Whereas modus-vivendi works
better when there is not a lot of sunshine or the room has a source of
light that is preferably a faint and/or warm one. It is possible to use
modus-operandi at night and modus-vivendi during the day, though that
will depend on several variables, such as one’s overall perception of
color, the paint on the walls and how that contributes to the impression
of lightness in the room, the sense of space within the eye’s peripheral
vision, hardware specifications, and environmental factors.
In general, an additional source of light other than that of the monitor can help reduce eye strain: the eyes are more relaxed when they do not have to focus on one point to gather light.
The monitor’s display settings must be accounted for. Gamma values, in particular, need to be calibrated to neither amplify nor distort the perception of black. Same principle for sharpness, brightness, and contrast as determined by the hardware, which all have an effect on how text is read on the screen.
There are software level methods on offer, such as the XrandR utility for the X Window System (X.org), which can make gamma corrections for each of the three channels of light (red, green, blue). For example:
xrandr --output LVDS1 --brightness 1.0 --gamma 0.76:0.75:0.68
Typography is another variable. Some font families are blurry at small point sizes. Others may have a regular weight that is lighter (thinner) than that of their peers which may, under certain circumstances, cause a halo effect around each glyph.
The gist is that legibility cannot be fully solved at the theme level. The color combinations may have been optimized for accessibility, though the remaining contributing factors in each case need to be considered in full.