See https://github.com/zsh-users/zsh-syntax-highlighting/issues/677#issuecomment-599225740 for details. (In particular, there's already another test that calls itself #616.)
zsh-syntax-highlighting / highlighters
Navigate into the individual highlighters' documentation to see
what styles ($ZSH_HIGHLIGHT_STYLES keys) each highlighter defines.
Refer to the documentation on highlighters for further information.