The fallback style name uses '_' in anticipation of a future auto-fallback feature keyed on style names. 'arg0' was previously known as 'commandtypefromthefuture'.
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.