How to create color shades using CSS variables similar to darken() of SASS?

Question

I'm looking a way of modifying a CSS variable as you would in SCSS

Define a color like primary - and automatically I would get shades for focus and actives states. Basically, would like to change one variable in css variables and get 3 shades of the same color.

What Id like to achieve in CSS

$color-primary: #f00;

.button {
    background: $color-primary;

    &:hover,
    &:focus {
        background: darken($color-primary, 5%);
    }

    &:active {
        background: darken($color-primary, 10%);
    }
}

trying to achieve:

:root {
    --color-primary: #f00;
    --color-primary-darker: #f20000  //     var(--color-primary) * 5% darker
    --color-primary-darkest: #e50000 //     var(--color-primary) * 10% darker
}

.button {
    background: var(--color-primary);
}

.button:hover,
.button:focus {
    background: var(--color-primary-darker);
}

.button:active {
    background: var(--color-primary-darkest);
}

Answer 1

The new Specification introduces "relative color syntax" where you can do the following

:root {
    --color-primary: #f00; /* any format you want here */
    --color-primary-darker: hsl(from var(--color-primary) h s calc(l - 5%)); 
    --color-primary-darkest: hsl(from var(--color-primary) h s calc(l - 10%)); 
}

The idea is to convert the main color to hsl format and using calc() you adjust the lightness.

There is still no support for this to date so consider the below solution.

---

You can consider hsl() colors and simply control the lightness:

:root {
    --color:0, 100%; /*the base color*/
    --l:50%; /*the initial lightness*/
    
    --color-primary: hsl(var(--color),var(--l));
    --color-primary-darker: hsl(var(--color),calc(var(--l) - 5%));
    --color-primary-darkest: hsl(var(--color),calc(var(--l) - 10%)); 
}

.button {
    background: var(--color-primary);
    display:inline-block;
    padding:10px 20px;
    color:#fff;
    cursor:pointer;
}

.button:hover,
.button:focus {
    background: var(--color-primary-darker);
}

.button:active {
    background: var(--color-primary-darkest);
}

<span class="button">some text</span>

As a side note, darken() is also doing the same thing:

Makes a color darker. Takes a color and a number between 0% and 100%, and returns a color with the lightness decreased by that amount.

Answer 2

How about this (pure sass/scss):

First, we need to split a color into hsla values and save each one in a separate custom property. Luckily sass has some functions to do the job.

@mixin define-color($title, $color) {
    --#{$title}-h: #{hue($color)};
    --#{$title}-l: #{lightness($color)};
    --#{$title}-s: #{saturation($color)};
    --#{$title}-a: #{alpha($color)};
}

Now we can put it back together, making some adjustments on the way.

@function color($title, $hue: 0deg, $lightness: 0%, $saturation: 0%, $alpha: 0) {
    @return hsla(
        calc(var(--#{$title}-h) + #{$hue}), 
        calc(var(--#{$title}-s) + #{$saturation}),
        calc(var(--#{$title}-l) + #{$lightness}),
        calc(var(--#{$title}-a) + #{$alpha}),
    );
}

Now we are ready to define some color variables...

:root {
    @include define-color("primary", #696969);
    @include define-color("secondary", blue);
}

override them (to dynamically switch between themes for example)...

:root.theme-light {
    @include define-color("primary", #424242);
    @include define-color("secondary", red);
}

use and adjust them!

.example-class {
    color: color("primary");
    background: color("secondary", $lightness: +20%, $alpha: -0.3);
    border: 1px solid color("primary", $hue: -30deg, $saturation: 5%);
}

Answer 3

If you are willing to take a different approach to your problem, using masks with the pseudo ":before" element would solve your problem. Although if you use this, i would advice you to put any content in the button inside a span or something, to give it a "z-index:1", so the content is not behind the mask.

:root {
    --color-primary: #f00;
}

.button {
    position:relative;
    background: var(--color-primary);

    &:before {
        content:'';
        position:absolute;
        width:100%;
        height:100%;
        top:0;
        left:0;
    }
}

.button:hover:before,
.button:focus:before {
    background:rgba(0,0,0,0.05) /* black mask with 5% opacity */
}

.button:active:before {
    background:rgba(0,0,0,0.1) /* black mask with 10% opacity */
}