Header colors in dark mode

[cisaacstern]

Over in pangeo-forge/pangeo-forge-recipes#610 I've noted that this theme's headers are not a sufficiently contrasting color when the page is rendered in dark mode, e.g.:

Looks like it's the !importants here causing an issue:

pangeo-sphinx-book-theme/pangeo_sphinx_book_theme/static/custom.css

Lines 5 to 17 in 0a0e20a

	h1 {
	font-family: "Panton Black Caps", sans-serif;
	color: #003B71 !important;
	}
	h2 {
	font-family: "Panton Light Caps", sans-serif;
	color: #322173 !important;
	}
	h3 {
	color: #322173 !important;
	}