Accessibility: Is there an easy way to allow user to change font size for body?

Morning all,

I have been searching for a way to change the font size on my single page’s body via CSS but so far have been unsuccessful. I have seen many sites that allow this – for example: https://www.latimes.com/news/local/la-me-malibu-fertilizer19-2010apr19,0,265717.story

some have A, A+ A++ icons

Can anyone point me in the right direction? I only want it for body content though.

Ideally, not a plugin.

Many thanks,
Fiona