theme property

A collection of attributes for the button. The object takes SVG attributes like fill, stroke, stroke-width or r, the border radius. The theme also supports style, a collection of CSS properties for the text. Equivalent attributes for the hover state are given in theme.states.hover.

In styled mode, drill-up button styles can be applied with the .highcharts-drillup-button class.

API Docs: https://api.highcharts.com/highcharts/drilldown.drillUpButton.theme