TornadoFX: Type-Safe CSS with other libraries

You mention the classes jfx-decorator and jfx-decorator-buttons-container, but your example CSS uses the classes jfx-decorator-buttons and container. I'm unsure which classes you really want, but I'll add the two latter, since that will produce the CSS from your example.

class Styles : Stylesheet() {
    companion object {
        val jfxDecoratorButtons by cssclass()
        val container by cssclass()
    }

    init {
        jfxDecoratorButtons and container {
            backgroundColor += Color.RED
        }
    }
}

UPDATE: You changed the code in your question, so here is the updated version that will produce that output:

class Styles : Stylesheet() {
    companion object {
        val jfxDecoratorButtonsContainer by cssclass()
    }

    init {
        jfxDecoratorButtonsContainer {
            backgroundColor += Color.RED
        }
    }
}

Camel cased selectors are automatically converted to lower cased with hyphens. You can also specify the exact name inside the cssclass delegate function:

val myCssClass by cssclass("my-slightly-different-css-class")

Notice also that since the backgroundColor property accepts multiple values, you must "add" the color to the list of colors using +=. This is the common pattern for all properties that accept multiple values.