Hi there,
The vertical line is Border style added to the Headline Block, you can change the border in the Headlines > Spacing >>> Border Width and the color of it in Headlines > Colors > Border color.
If you select a string of words in the Headline block content, and go to the tool bar drop down arrow and select Highlight, this wrap those words in a <mark>
tag which can then be styled in Colors > Highlight.
If you want to add custom style to the Headline, then first give the Headline Block a CSS class in Advanced > Additional CSS Class(es) eg. my-custom-class
you can then style the highlight with CSS:
.my-custom-class mark {
/* Custom styles for the Headline Mark */
}