The default background was gray for Netscape, even on Windows, presumably because that was the default background for common Unix toolkits like Motif/X. Even Internet Explorer defaulted to gray. See these Wikipedia articles containing screenshots:
In particular, notice the IE2 screenshot, which shows it displaying the more modern Wikipedia website using a gray background. IE2 doesn't understand CSS, so just displays the source. The CSS declares a white background, so we can infer that the HTML probably does not declare a gray background using the long forgotten BODY tag BGCOLOR attribute. (Rather, almost certainly the HTML doesn't use the BGCOLOR attribute at all.)
It's possible that Mosaic defaulted to white on Mac OS, but I doubt it. Netscape didn't back then, IIRC.
For a long time gray and gray-toned backgrounds were ubiquitous on the web. It was nice because white backgrounds are difficult on the eyes, especially for prolonged periods. Unfortunately, white eventually began to dominate, as it already did for most Windows and Mac applications. Now we're coming full circle with dark theming, though dark theming is typically much darker than the old web and old X applications.
https://en.wikipedia.org/wiki/ViolaWWW https://en.wikipedia.org/wiki/Mosaic_(web_browser) https://en.wikipedia.org/wiki/Internet_Explorer_2 https://en.wikipedia.org/wiki/Netscape_Navigator
In particular, notice the IE2 screenshot, which shows it displaying the more modern Wikipedia website using a gray background. IE2 doesn't understand CSS, so just displays the source. The CSS declares a white background, so we can infer that the HTML probably does not declare a gray background using the long forgotten BODY tag BGCOLOR attribute. (Rather, almost certainly the HTML doesn't use the BGCOLOR attribute at all.)
It's possible that Mosaic defaulted to white on Mac OS, but I doubt it. Netscape didn't back then, IIRC.
For a long time gray and gray-toned backgrounds were ubiquitous on the web. It was nice because white backgrounds are difficult on the eyes, especially for prolonged periods. Unfortunately, white eventually began to dominate, as it already did for most Windows and Mac applications. Now we're coming full circle with dark theming, though dark theming is typically much darker than the old web and old X applications.