Some Ext JS container exposes CSS variables without any mixin. For example, `fieldcontainer`. In my custom theme I want to style two `fieldcontainer`s differently using the available CSS variables for `fieldcontainer`.

I know it can be done by applying CSS. Is there a way to achieve it by setting the CSS variables?

For example,

```
    .my-class-one {
        $form-label-font-color: #FFFFFF
    }

    .my-class-two {
        $form-label-font-color: #000000
    }

```

Is it possible? If possible, where do I put this code?