I think the default styles are OK; I do not think they are so bad as some people say. However, the thing I would add (for web pages that have pictures) would be:

  img { max-width: 100%; }

(which is one of the things mentioned in that article; it probably would make sense for videos and SVG as well like they mention.) But, if you don't like them, the user should be allowed to customize them. In case this causes problems with web pages that use different styles that interfere with them, then hopefully an attribute could be added to specify that the user's CSS should be used instead of this one (and in some cases the browser might be able to decide this automatically for when the web page does not specify this attribute, e.g. if the styles are only associated with media queries and HTML element names rather than IDs, classes, etc). The user could still override this to disable the web page's CSS entirely if desired, but this would make it possible to specify that the web page's CSS is not needed for styling classes and other more complicated stuff within the specific web page or web app. (If the user does not define their own CSS, then it would use that defined in the web page (or the defaults if there is no CSS in the web page; the defaults probably should be changed to support the light/dark scheme properly for web pages that lack CSS, though), and the hypothetical attribute that I mentioned would be ignored.)