Change Heading sizes in Native template?

The Native template only has one Heading size. How can I edit the sizes of H1, H2, and H3?

I tried this in the Post Blog Item Code Injection, but it did not work:

<style> h1 { color: _; font-size: _px; }

h2 { color: _; font-size: _px; }

h3 { color: _; font-size: _px; }

p { color: _; font-size: _px; } </style>