I noticed within the Formatting options for any column charts, there is no option available to customise the border colour. Within Formatting > Data Colors a user can customise the 'fill' colour, but not the border. This option isn't available within any of the other formatting sections either. It looks like another user requested this previously, but couldn't find a related submission: https://community.powerbi.com/t5/Desktop/Format-Change-style-of-border-of-a-column-in-a-quot-line-and/td-p/1023446