Hi there,
you need your own image for the background then:
1. Add this CSS to your site:
.custom-highlight mark {
position: relative;
}
.custom-highlight mark::before {
content: '';
position: absolute;
bottom: 0;
left: 0;
width: 100%;
height: 1em;
background-image: url('your_image_url');
background-repeat: no-repeat;
}
Replace the your_image_url
with your images full URL.
2. Then on your page, add a Headline block with your text. Hightlight the word you want the background for and in the block toolbar 3 dot menu select Highlight ( the top icon ).
3. With the Headline block still selected, in Advanced > Additional CSS Class(es) add: custom-highlight
You will need to make sure you have an image that is the appropriate proportions for the text you want to highlight.