AO3 Skins

You'd need to start by figuring out how to make a paragraph look like those bubbles - what colour background, colour text, border colour and width, rounded corners, etc. Then add a class for SMS, that you can add to appropriate paragraphs in the story, that will trigger that CSS from the workskin. If you're new to CSS, the W3Schools site isn't perfect and teaches some bad long-term habits but will cover all that stuff in a beginner-friendly way.

(I have not made a workskin in years and may be misremembering, but that's how I'd start.)