Let Verticator know this by setting 'darktheme' to true.
Reveal.initialize({
...
verticator: {
darktheme: true
},
plugins: [ Verticator ]
})
This slide has a light data-background attribute (while the theme is dark). The bullets will be black.
This slide has a dark data-background attribute.
It has no effect on the Verticator bullets, because the theme is already dark.