Bulma CSS Vars

Bulma CSS Vars extends Bulma to use CSS variables, that can be set to arbitrary values at runtime on the website and installs fallbacks for browsers without CSS variables capabilities.

This is an extension and a kind of "sass-pre-post-processor" that tries to be as least intrusive as possible to Bulma, while making arbitrary color changes in the bulma color schemes automated, as easy as possible.

There is quite some setup and configuration to be done, but once it is setup, it works like a charm. Read the section What is the difficulty, to learn why this setup is required.

Usage

 npm i -S bulma bulma-css-vars
npm i -D sass
node ./node_modules/.bin/bulma-css-vars --init

To use this package, you have to use the dart implementation of sass, the sass package, version 1.23 or higher. If you use webpack and the sass-loader, you have to make sure you do not have node-sass installed, or configure options: { implementation: require('sass') } for the sass-loader.

 // bulma-css-vars.config.js
import { hsl, rgb } from 'bulma-css-vars'

// color names have to match bulma's $variable-name, without '$'
// values will be used for initial colors and fallback
const appColors = {
  black: hsl(0, 0, 4),
  'scheme-main': rgb(200, 105, 84),
  red: { r: 255, b: 0, g: 0}
  primary: '#663423',
  blue: 'blue',
}
// reuse variable colors
appColors['text'] = appColors['primary']

export default {
  sassEntryFile: './src/style/main-sass-file.sass',
  jsOutputFile: './src/bulma-generated/bulma-colors.js',
  sassOutputFile: './src/bulma-generated/generated-vars.sass',
  cssFallbackOutputFile: './src/bulma-generated/generated-fallbacks.css',
  colorDefs: appColors,
  globalWebVar: false,
  transition: '0.5s ease',
}

You need to configure bulma-css-vars to tell it about your sass setup, especially your sass entry file, the variables you want to modify and where the generated bulma files should be placed.

Some more files have to be setup, which can be achieved using node ./node_modules/.bin/bulma-css-vars --init.

 // main.scss
@import './bulma-generated/generated-fallback.css'; // import fallbacks first, so they are overridden
@import './bulma-generated/generated-vars.sass';
@import '../node_modules/bulma-css-vars/bulma-cv-lib';

Instead of using bulma-cv-lib.sass, you can also just use the bulma packages you like. Look inside the bulma-cv-lib.sass to understand more, and especially import functions.sass right after the original functions.sass is loaded.

 // package.json
  scripts: {
    "update-bulma-colors": "bulma-css-vars",
  }

The script ./node_modules/.bin/bulma-css-vars has to be run whenever you modify the colors in bulma-css-vars.config.js and it will update the three output files as well.

 // in-the-web-app.js
import { ColorUpdater } from 'bulma-css-vars'
import { bulmaCssVariablesDefs } from './bulma-generated/bulma-colors'

const updater = new ColorUpdater(bulmaCssVariablesDefs)

// do the update manually
const updated = colorUpdater.getUpdatedVars(colorName, value)
updated.forEach(({ name, value }) =>
  document.documentElement.style.setProperty(name, value)
)
// or let it do the updater
colorUpdater.updateVarsInDocument(colorName, value)

You can also use TypeScript

 // in-the-web-app.ts
import { ColorUpdater } from 'bulma-css-vars'
import { bulmaCssVariablesDefs } from './bulma-generated/bulma-colors'

// the updater do the style change
const colorUpdater = new ColorUpdater(bulmaCssVariablesDefs)
colorUpdater.updateVarsInDocument(colorName, value)

Annoyingly, the color updater needs knowledge of the current variables, so bulmaCssVariablesDefs from the generated js file has to be included in your app. colorName has to match the name in the bulma-css-vars.config.js.

Caveats

• Requires Node.js and Dart Sass
• The complexity of the setup

What is the difficulty? Why this setup?

The problem is, that Bulma relies heavily on sass preprocessing, so if you set a color variable for $primary, this color will be modified for hover / focus shades on buttons, inversions for texts on buttons are calculated, and so on.
If we would inject a CSS variable var(--my-dynamic-primary-color) here, not only will sass fail, also if you will change this color on runtime, still all the computed shades will remain calculated from the initial color.

The solution - how it works

First off this library patches original sass functions such as darken, lighten, as well as bulma helper functions like findColorInvert. This is currently only possible thanks to the power of the dart sass package, which has more powerful possibilities than the node-sass package. The patched functions can handle var(--variablename) variables and decide if the original function will be called or if a new, derived variable of the incoming variable gets added.

For example, darken(var(--primary), 10%) will turn into var(--primary--10--darken), and a derived variable gets added to the css stylesheets.
This library automates this process, so for each actually used and derived variable a new css variable gets created. To leverage this technique, the library then compiles the css and gets an overview of which derived variables are all in the actually used stylesheets.

The advantage of this technique is that only actually required variables will be generated and added to the stylesheets. The disadvantage is, that you need to run this command whenever you decide to add an additional variable to your app.

From the list of base variables and derived variables, this library then can create an initial sass file, that you should include in your sass compilation. It calculates the style using a js version of darken, lighten, findColorInvert. As an example, --primary--10--darken: #532949; would be generated by bulma-css-vars as well, next to --primary: #694392;.

The class doing this should also be included into your application. Whenever you want to change a bulma color variable, e. g. say $blue, the color updater knows which derived variables are required to be adjusted as well and will let you update all affected variables.

This way you can keep using the full bulma color richness. Try out the demo and see how the font of the buttons changes on dark / bright colors!

Direct Web Setup

If you do not use any bundler or web framework, you can also include bulma-css-vars directly. You will still require Node.js and sass.

// app.sass
@import './bulma-vars.sass'
@import './node_modules/bulma-css-vars/bulma-cv-lib.sass'

Make sure you set globalWebVars to true in your bulma-css-vars.config.js. Then run ./node_modules/.bin/bulma-css-vars and sass app.sass > app.css.

In your html:

 <head>
  <link res="stylesheet" src="./app.css">
</head>
<body>
  <!-- [your webpage] -->

  <!-- loads window.BulmaColorUpdater -->
  <script src="./node_modules/bulma-css-vars/dist/bulma-css-vars.web-bundle.js"></script>

  <!-- loads window.bulmaCssVarsDef with your variable definitions -->
  <script src="./bulma-colors.js"></script>
  <script>
    const updater = new BulmaColorUpdater(bulmaCssVarsDef)
    updater.updateVarsInDocument('black', '#553292')
  </script>
</body>

New Features

PRs are welcome!
If you want to build this repository and try out the demo, make sure you use yarn and yarn install to install the packages in the demo folder. npm will create symlinks instead of true file copies, which might create problems on referencing relative bulma files.