gnus-visual-mark-article-hook

gnus-visual-mark-article-hook is a variable defined in `gnus-sum.el'.
Its value is

(gnus-highlight-selected-summary)

• This variable may be risky if used as a file-local variable.

Documentation:
*Hook run after selecting an article in the summary buffer.
It is meant to be used for highlighting the article in some way. It
is not run if `gnus-visual' is nil.

You can customize this variable.