Here’s a link to the theme, for others’ convenience: Magma Theme: A dark theme for Obsidian
It looks like the headers aren’t explicitly selected in the theme, which is why you can’t find ‘em—they’re being styled implicitly through the other selectors.
So, all you should need to do is:
h3 {
color: orange;
}
Or, if you want it to be the same colour as the rest of the theme:
h3 {
color: var(--text-accent);
}
…I think. I’m bad at CSS when I’m not guessing and checking…