merge method
Copy this with a new typography
Implementation
Typography merge(Typography? typography) {
if (typography == null) return this;
return Typography(
header: typography.header ?? header,
subheader: typography.subheader ?? subheader,
title: typography.title ?? title,
subtitle: typography.subtitle ?? subtitle,
base: typography.base ?? base,
body: typography.body ?? body,
caption: typography.caption ?? caption,
);
}