Update styling.md for CSS custom properties clarification

[gunayanil]

I think the current note is a bit confusing/misleading if I understand it correctly. It indicates that a CSS custom property must first be declared on :host to be exposed to the consuming application, but I don't think this is the case.

For example, this works without declaring the CSS custom property on :host:

:host {
  display: block;
}

.heading {
  color: var(--heading-color, black);
}

Even color: var(--heading-color); should work fine too.

Then in consumer:

shadow-card {
  --heading-color: blue;
}

Here, shadow-card is the host element, and the CSS custom property inherits into the shadow tree without first being declared on :host.

[vercel]

@gunayanil is attempting to deploy a commit to the Ionic Team on Vercel.

A member of the Team first needs to authorize it.