How to apply style modification to a Horizontal Stack Card Title which is nested into a Vertical Stack Card?

Hi All !

I’d like to modify the spacing and font size of the title of a Horizontal Stack Card which is nested into a Vertical Stack.
I can modify the Vertical Stack title with the below code, but I can’t figure out how to make this work with the nested Horizontal Title.

Please help, the below code is working for the Vertical Title and and the target would be to apply the same on the horizontal title (or only to that).

type: custom:mod-card
style:
  hui-vertical-stack-card$: |
    .card-header {
      font-size: 11px !important;
      margin-top: -20px !important;
      margin-bottom: -15px !important;
      margin-left: -10px !important;
    }
card:
  type: vertical-stack
  title: Vertical Title
  cards:
    - type: horizontal-stack
      title: Horizontal Title
      cards:
        - type: custom:digital-clock
          dateFormat: yyyy/LLLL/dd
          timeFormat: HH:mm:ss