What problem does this address?

Currently, it's possible to change with the highlight features text or background color. Because the possibilities to customize this are very limited, it'd be great if we would add styles as we know it from block styles.

What is your proposed solution?

Add the option to add (block) styles for highlight / the <mark> tag, which are then selectable in the UI.

Example for a custom style:

.is-style-highlight-yellow {
/** just a example class name **/
    background: 0 0;
    background-image: linear-gradient(90deg, rgba(255, 225, 0, .1), rgba(255, 225, 0, .7) 4%, rgba(255, 225, 0, .3));
    border-radius: .8rem .3rem;
}

Example taken from: https://www.commercegurus.com/product/shoptimizer/