some text using a custom color property, in a section using another custom color property for the background-color.

An example

The subheading uses the same custom property for the color, with a border selector using the one of the custom properties used above for the color value of the border property.

The CSS is shown below:

html {
  --darkGreen: #5c8;
  --myPink: #fce;
  --myBorder: 4px solid var(--myPink);
  --myCodeColor: #88f;
}

section {
  color: var(--darkGreen);
  background: var(--myPink);
}

article h3 {
  color: var(--darkGreen);
  border-bottom: var(--myBorder);
}

code {
  color: var(--myCodeColor);
}
Syntax-coloured HTML generated using hilite.me