In the element theme, the font-size in :root should be scoped to hydrogen

To avoid bleeding outsize of the hydrogen container when hydrogen is embedded in a web page.

https://github.com/vector-im/hydrogen-web/blob/25d0cbe9a2120169db2e00942d216b31afbcf021/src/platform/web/ui/css/themes/element/theme.css#L21

Start with src/platform/web/ui/css/themes/element/theme.css at the linked :root rule and inspect how the hydrogen container is represented in the theme styles. Done means the font-size no longer bleeds outside an embedded hydrogen container while preserving the intended sizing inside it.

Written by the indexing model from the issue text.