Foreground and background color

Hello everyone,

I don't understand a failure of success criterion F24
<https://www.w3.org/WAI/WCAG21/Techniques/failures/F24>.

As I undesrtand it, it is necessary for the author of a document to define
the foreground and background colors of elements directly or inherited.
This avoids color contrast problems that would be caused by the user
customizing the colors in the user agent.

In the description it is specified:
"Users with vision loss or cognitive, language and learning challenges
often prefer specific foreground and background color combinations. In some
cases, individuals with low vision will find it much easier to see a Web
page that has white text on a black background, and they may have set their
user agent to present this contrast. Many user agents make it possible for
users to choose a preference about the foreground or background colors they
would like to see without overriding all author-specified styles. This
makes it possible for users to view pages where colors have not been
specified by the author in their preferred color combination."

How can an author, who needs to customize colors, do so if the author's
styles overwrite the user's styles?

Thank you for your answers.

--
Steven Mouret