Learning 11ty's Data Cascade by Making a Random Site Theme Generator

11ty has this excellent concept of the Data Cascade, which will be familiar to users of CSS. One element of that Cascade is that any JavaScript (or JSON) you place within the /_data directory of your project will be read at build time, and if the JavaScript exports an object it will be included within a global data object as a parameter with the same name as the file. JSON files get rolled in in the same manner. You can then use that data within your template files with fileName.parameterName.

To try out this concept while building my new portfolio site, I decided to use it to randomly generate the two colors that make up the theme of my site.

The first file this uses is /_data/theme.js, which defines two random colors and builds an object with two values we'll use in our CSS.

// set a random hue, then get the hues 100 and 200deg away from it
const randColor = Math.round(Math.random()*360)
colors = [randColor, (randColor + 100) % 360, (randColor + 200) % 360]

// build an object with paramaeters for each
// of the three colors, each an object with
// a hue and a brightness
const colorExports = {}
colors.forEach((color, i) => {
    colorExports[`color${ i+1 }`] = {}
    colorExports[`color${ i+1 }`].hue = color + 'deg'
    colorExports[`color${ i+1 }`].brightness = rgbBrightness(hsl2rgb(color/360, .5, .5)).toPrecision(3)

module.exports = {

// helper function for hsl2rgb
function rgbBrightness(rgb) {
    return Math.sqrt(
        rgb.r**2 * .241 +
        rgb.g**2 * .691 +
        rgb.b**2 * .068

function hsl2rgb(h, s, l) {
    // ...do some conversions i googled
    return { r, g, b }

The only other thing we need is an include called theme.njk (I used Nunjucks for my website but can use a number of template languages). Includes are snippets of code that you can save as separate files within an /_includes directory, then drop into your template files. Nunjucks is usually used for HTML, but this file will just be a style tag so we can use Nunjucks' superpowers in our CSS.

<!-- /_includes/theme.js -->
<style id='theme-data'>
   {% raw %}
    :root {
        {% for colorName, color in theme %}
        --theme-hue-{{loop.index}}: {{ color.hue }};
        --theme-brightness-{{loop.index}}: {{ color.brightness }};
        --theme-color-{{loop.index}}: hsl(var(--theme-hue-{{loop.index}}), 60%, 85%);
        --theme-color-{{loop.index}}-dark: hsl(var(--theme-hue-{{loop.index}}), 80%, {% adjustBrightness 5, 20, color.brightness %}%);
        {% endfor %}
    {% for colorName, color in theme %}
    a.theme-{{loop.index}}:focus {
        color: hsl(var(--theme-hue-{{loop.index}}), 60%, 65%);
    a.theme-{{loop.index}}:hover path,
    a.theme-{{loop.index}}:focus path {
        fill: hsl(var(--theme-hue-{{loop.index}}), 60%, 65%);
    {% endfor %}
    {% endraw %}

In this file we loop over each of the colors created in our theme.js file, which is available simply as theme, and create a series of CSS custom properties we can then use throughout our site. Just for fun I added some classes that give links with a theme class a hover color too.

Now I include this include into my base.njk template that every page inherits from.

<!-- /_includes/layouts/base.njk -->
<!doctype html>
<html lang="en">
  <!-- ... more meta and styles ... -->
  {% raw %}
  {% include theme.njk %}
  {% endraw %}
  <!-- more HTML and whatnot -->

And just like that I've got a bunch of theme CSS variables available for me to color things across the site with which are randomly determined at build time! I use it to color the wave SVG on the home page, and theme-color-1 is used for all my writing while theme-color-2 represents all my work.

I never know what colors my site will have until after I rebuild it, which makes the process just a bit more fun. Currently theme color 1 is {{ theme.color1.hue }} and color 2 is {{ theme.color2.hue }}. And if I don't like them, well that's just that little nudge I need to write my next post.