Watch the tutorial: https://youtu.be/hKGX1jRpp6I

1-basics.css

h1, h2, h3, h4, h5, h6 {
	font-weight: bold;
}

:is(h1, h2, h3, h4, h5, h6) {
	font-weight: bold;
}

2-original.css

.bg-primary-dark,
.bg-secondary,
.bg-secondary-dark,
.bg-primary-dark h1,
.bg-primary-dark h2,
.bg-primary-dark h3,
.bg-primary-dark h4,
.bg-primary-dark h5,
.bg-primary-dark h6,
.bg-secondary h1,
.bg-secondary h2,
.bg-secondary h3,
.bg-secondary h4,
.bg-secondary h5,
.bg-secondary-dark h6,
.bg-secondary-dark h1,
.bg-secondary-dark h2,
.bg-secondary-dark h3,
.bg-secondary-dark h4,
.bg-secondary-dark h5,
.bg-secondary-dark h6,
.bg-primary-dark a:not(.btn),
.bg-secondary a:not(.btn),
.bg-secondary-dark a:not(.btn) {
	color: #ffffff;
}

.bg-primary-dark figcaption,
.bg-secondary figcaption,
.bg-secondary-dark figcaption {
 color: #f7f7f7;
}

.bg-primary-dark a:not(.btn):hover,
.bg-secondary a:not(.btn):hover,
.bg-secondary-dark a:not(.btn):hover {
	color: #d9d9d9;
}

3-is.css

.bg-primary-dark, .bg-secondary, .bg-secondary-dark,
:is(.bg-primary-dark, .bg-secondary, .bg-secondary-dark) :is(h1, h2, h3, h4, h5, h6, a:not(.btn)) {
	color: #ffffff;
}

:is(.bg-primary-dark, .bg-secondary, .bg-secondary-dark) figcaption {
 color: #f7f7f7;
}

:is(.bg-primary-dark, .bg-secondary, .bg-secondary-dark) a:not(.btn):hover {
	color: #d9d9d9;
}

4-nesting.css

.bg-primary-dark,
.bg-secondary,
.bg-secondary-dark {

	h1, h2, h3, h4, h5, h6,
	a:not(.btn) {
		color: #ffffff;
	}

	figcaption {
		color: #f7f7f7;
	}

	a:not(.btn):hover {
		color: #d9d9d9;
	}

}