To use the toggle theme simply you have to add the dependencies from the blank theme module into you reactive application and then follow the steps below to configure it.
So at first place we have two modules one is a sample and other one is a theme module from which you have to take dependencies
To Customize the look of your dark theme and if you want to change colours
You can customize it in a theme Module
Refer to this discription, if in case of any question feel free to contact.
https://www.outsystems.com/forge/component-overview/13516/outsystems-ui-dark-light-theme-toggle