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You could start with native CSS variables. :)

Not very easy to darken/lighten colours easily using CSS variables at the moment. Support will be coming in the future.

I find it quite easy. Just use the hsl() function.

I don't have anything to hand and I only use a tiny number of Sass's built-in features, but here's a super-quick demo of styling with variables and darken (https://sass-lang.com/documentation/modules/color#darken).

Note, darken and lighten aren't always favoured these days, it's good advice to learn scale (https://sass-lang.com/documentation/modules/color#scale), but darken or lighten makes for an easier demo.

Sass Basics seems like a good place to start: https://sass-lang.com/guide