How can I ensure more contrast when viewing HTML mail?
Gnus’ built-in simple HTML renderer (you use it if the value of
mm-text-html-renderer is shr) uses the colors which are
declared in the HTML mail. However, it adjusts them in order to
prevent situations like dark gray text on black background. In case
the results still have a too low contrast for you, increase the values
of the variables shr-color-visible-distance-min and
shr-color-visible-luminance-min.