Change caption on TextField focus - css

Question

In my Vaadin App I want to change the color of a focused TextField, which is no problem. Additionaly I want to change the color of the caption which belongs to the TextField. How can I achieve this with css?
.v-textfield.textfield-default {
border: none;
border-bottom: 1px solid $non-active-field;
outline: none;
height: 3rem;
font-size: 1rem;
margin: 0 0 15px 0;
padding: 0;
box-shadow: none;
box-sizing: content-box;
transition: all 0.3s;
}
.v-textfield.textfield-default:focus {
border-bottom: 1px solid $default;
}
.v-caption-default-caption {
color: purple; //changes the text to purple
top: 0.8rem;
left: 0.75rem;
font-size: 1rem;
cursor: text;
transition: .2s ease-out;
}
.v-caption-default-caption:focus {
color: red; //is never called
}
.v-caption-default-caption:active {
color: blue; //is never called either
}

Answer 1

Note: I'm not a CSS/SCSS guru thus more elegant solutions may exist that I'm unaware of. The only one I could come up is Vaadin-based (also java 8), but corrections and suggestions are more than welcome.
From what I gathered, the problem in this case is that you need to update the previous sibling of the input that gets focused, aka it's caption. I've done a bit of research and so far it does not seem possible with CSS.
Also looking at the DOM (see image below), only the text-field gets foucused, hence none of the styles you've defined for the caption gets applied. Under the circumstances, a quick workaround that you can use, is to add focus & blur listeners to your text-fields, which will add & remove a custom style you're also going to define.
Step 1: The component
public class MyTextFieldsComponent extends VerticalLayout {
public MyTextFieldsComponent() {
// the text-fields
TextField myFirstField = new TextField("My first caption");
TextField mySecondField = new TextField("My second caption");
// when focused, add our custom style
FieldEvents.FocusListener focusListener = event -> event.getComponent().addStyleName("red-caption");
// when blurred, remove the custom style
FieldEvents.BlurListener blurListener = event -> event.getComponent().removeStyleName("red-caption");
// use the above listeners
myFirstField.addFocusListener(focusListener);
mySecondField.addFocusListener(focusListener);
myFirstField.addBlurListener(blurListener);
mySecondField.addBlurListener(blurListener);
// add the text-fields to the UI
addComponent(myFirstField);
addComponent(mySecondField);
}
}
Step 2: The style
.v-caption-red-caption {
color: red;
}
Step 3: The result