There is a feature request with Kore.ai to enable dark-mode on bot-builder UI.
We are not offering this feature out of the box but as of now, bot builder works fine with “Care Your Eyes” plugin on Chrome. We neither advertise this plugin nor are responsible for issues with it. This is just an FYI for the users.
However to facilitate users, early May we are planning to make some more changes to bot-builder UI which supports java-script editor on the above plugin.
Note - As of now as per our information Care Your Eyes is available only on Chrome and it is at their discretion to make it available for other browsers.