How to pin custom in-app message fonts in AOS

I'm creating a custom in-app message that renders a constant font size even when the system font changes, but in AOS, the font of the in-app message changes depending on the system font size.

I know that in-app message is rendered in webview environment, so I wrote the code text-zoom : 100% inside the app, but I have the same problem.

Does anyone have a solution for this?