There is no special configuration needed. Just create a CssInjectDevice and place it somewhere in the GUI. It doesn't matter where. Create a rule that contains a CSS variable, a CSS selctor and an attribute that corresponds to the variable.
when dummy-switch is turned on then set css "color" of "#my-favorite-element" to "green"
This plugin is in an early alpha stadium and you use it on your own risk. I'm not responsible for any possible damages that occur on your health, hard- or software.