HI,
Those are css variables. In Outsystems UI Web if you look into the top of the theme you will see that a couple of css variables were declared. You use those variables or declare your own. This has nothing to do with the variables you use in your actions.
If you aren't familiarize with css variables you can just use this
document.documentElement.style.setProperty('" + <actionvariable> + "', 'font-size');
on the runjavascript action that you can find on HTTPRequestHandler module.
Regards,
Marcelo