The combination of green and red used in these pages is not an accident.
It is intentional.

The shades of red and green used in the text and background of these pages (but not this one) are very nearly equiluminant.  Although these two colors have very different hues, the overall brightness (luminance) of the this particular red and green combination is almost equal. However, the red and green in the page your are reading now are much less equiluminant. This is why the contrast is so much stronger on this page. On the other pages the low contrast in luminance is combined with a high contrast in hue.

This induces an unusual optical sensation in the human eye-brain system.

The human eye-brain system perceives color and brightness in very different ways. The part of the eye-brain that perceives hue (what most people refer to as color) is sometimes referred to as the "what system". The "what system" is responsible for object recognition and color perception. The part of the eye-brain system that perceives brightness (a fundamental determinant of color - but independent of hue) is sometimes referred to as the "where system". The "where system" is responsible for motion and depth perception, spatial organization and figure/background segregation.

Our eyes are accustomed to receiving corresponding signals from both the what and where systems simultaneously (because very few things in nature are equiluminant) so they tend to behave erratically when confronted with scenes that are dominated by objects with very different hues - but equiluminant brightnesses. This induces the optical unusual sensation that some people experience when they look at these pages.

Some people find this optical sensation pleasantly stimulating.
I hope you do.

If this kind of thing interests you, an excellent introduction to visual phenomena can be found in Margaret Livingstone's book "Vision and Art: The Biology of Seeing".