In the beginning html was described in a ten page document and the sites so rendered appeared with gray background in the browsers of the day. No longer.
Early Amazon.com website with gray background. twitter 
I thought the gray background to be an interesting decision of the early implementers. I was familiar with studies of the human visual system that suggested gray to be a better background choice on illuminated displays than either black or white. It would take some getting use to but I thought there could be long term benefit.
Not so.
As html documentation grew to uncountable pages so did the ability to specify the background color of any page. Gray took on a new meaning: incompetence.
A recent update aligns plugins with the emergent convention of flat gray backgrounds and centered labeling. github
Soon if one encountered a page with a gray background there would be only two possible explanations: either the page was so old that one should seek a newer one, or, the new page author was so unfamiliar with web technology that they were unable to make their pages white.
There also emerged the cult of the black background where those inhabiting dim rooms complained that white burned their eyes.
And then there were the ghastly pink, yellow, purple and cyan backgrounds which demonstrated the disconnect between competence and taste.
In wiki we choose white background for text and a light gray background for mechanisms.