Grid Header css

Trying to apply css to highlight a grid header (I am trying out vaadin-grid-flow:1.2.1), but I can’t seem to find the right CSS rule.
Browser inspector shows that both header and body cells have the same type:

You need a style/theme module to change the styles of the elements inside the grid’s shadow DOM (the ones with a part attribute are part of the public styling API). Here’s a quick example:

<dom-module id="my-grid-styles" theme-for="vaadin-grid">
		  /* Highlight styles here */

Read more about the styling mechanism here:

Thanks, got it.