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Less and Sass are CSS “pre-processors”: they include higher level functionality like variables, functions, and nesting on top of CSS and can then be compiled to CSS.


As of this writing, all VuFind® style development is done using LESS; SASS files are programmatically derived from the LESS files using the lessToSass NPM script. While LESS and SASS files should be functionally equivalent, the LESS files are more thoroughly tested and we apologize for any problems with SASS. Please feel free to contribute back to the project if you wish to improve our support of SASS.

Command Line Compilation

Both Less and Sass use NPM scripts for compilation. They both compile to css/compiled.css.

LESS Theme Inheritance

This is a bit tricky and is still a problem for VuFind®; we are especially open to help/suggestions for making this more straightforward and easier to understand.

CSS inheritance is pretty straight forward in VuFind®: name your CSS file the same if you want to override the parents', different if you want to extend. In LESS, you can import other files into your stylings, which is VERY useful for things like the Bootstrap library because of all the variables. Being able to use those variables to color and size things correctly is crucial.

In order to utilize the power of imports and the power of inheritance, here's what's going on in the Bootstrap 3 and Bootprint 3 themes.


  1. compiled.less (only imports bootstrap.less, included in theme.config.php)
    1. @import “bootstrap”;
  2. bootstrap.less (contains all customizations, ala bootstrap-custom in Bootstrap2)
    1. @import “bootstrap/bootstrap”;
    2. @import “a11y”;
  3. a11y.less (Accessibility theme, changes variables for color/padding, high contrast hovers, .sr-only class)


  1. compiled.less (pulled in instead of bootstrap3/compiled.less, due to inheritance)
    1. @import “bootprint”;
  2. bootprint.less (contains all custom stylings for Bootprint 3, ala bootprint-custom in Bootprint2)
    1. @import “bootstrap”; ← parent theme.less
    2. @import “variables”;
    3. @import “icons”;
  3. variables.less (tweaking of Bootstrap variables, imported into bootprint.less)
  4. icons.less (FA icon overwrites, included in theme.config.php)

Make sure you have a compiled.less in your theme, as the LESS compiler may have trouble pulling from parent themes. Also, doing so will prevent multiple Bootstrap instances from being compiled into your final CSS.

:!: This feature was removed in VuFind® 9.0, and its use is strongly discouraged – the built-in PHP-based LESS compiler did not generate output consistent with the Node.js LESS compiler, and this had a high potential for causing problems.

Active compilation can be used to automatically compile from Less to CSS when a page loads. This is very useful for development.

Write permission needs to be given to a new folder for LESS to work: [theme]/css/less/

Configure in theme.config.php:

"less" => array(
  "active" => true/false,

For now, every LESS theme needs a compiled.less which “@import”s a custom theme.less or a parent.less.

:!: Note that the build:css NPM script is the preferred means of compiling LESS files for VuFind®, and the older PHP compiler may produce inconsistent results in some situations. The PHP compiler is deprecated as of VuFind® 8.0 and was removed in VuFind® 9.0.

php util/cssBuilder.php

This will use the Less Parser to compile your LESS into CSS files in the regular CSS folder. These can then be used as regular CSS files, but more importantly, these will be used as fall-back in case of a LESS parser error. So compile regularly!

LESS does not have to be set to active for this to work, as long as the files you want to compile are listed under “less” in theme.config.php.

development/architecture/less.txt · Last modified: 2023/04/20 15:50 by demiankatz