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LESS (lesscss.org) extends CSS with dynamic behavior such as variables, mixins, operations and functions. It's an advanced way of writing CSS that we use in some themes within Moodle.

Browsers don't interpret it themselves, however, so LESS files need to be converted into normal CSS before use.

Recess is one tool used to compile and compress LESS into CSS under *nix based systems. There are many others tools for LESS.

The bootstrap theme

Moodle's Bootstrap base theme has rules written using LESS.

The main LESS file is theme/bootstrap/less/moodle.less, with additional files in theme/bootstrap/less/moodle/ imported by the main file.

After compiling the LESS files, CSS ends up in theme/bootstrap/style/generated.css. This file should not be edited manually.



Installing Recess under Ubuntu

The following commands should be run to install Recess

1. sudo apt-get install npm node
   (if the above fails due to the error "The following packages have unmet dependencies", run the commands separately, ie. sudo apt-get install npm and then sudo apt-get install node)
2. npm install recess -g
stands for Node Package Manager, and is the equivalent of apt-get for node.js packages.

Installing Recess on Mac OS X

1. Install the basic node packages from the web site: http://nodejs.org (or use your favourite package manager to install

2. On the command line, install recess like this:

    npm install recess -g

Using Recess

After editing the LESS files, compile your CSS as follows:

cd theme/boostrap/less/
recess --compile --compress moodle.less > ../style/generated.css

This will compile and compress the moodle.less file (and all the LESS and CSS files it imports) into a single file, generated.css, and store it in the style folder of the bootstrap theme.

NOTE: if you are getting empty generated.css files, this is being caused by a parsing error in your LESS code. A bug in
currently prevents it giving you helpful error messages in many cases. You will need to examine what you have altered or written to make sure it is complete and the syntax is correct. The
compiler is what is used by
to do the actual compiling and will have been installed automatically along with it. If you call it directly as
lessc moodle.less
it should give you a helpful error message that points you to where the problem is.


Download WinLess and install it.

Using WinLess

1. Click the Add folder button.

2. Select theme\bootstrap\less\ as the folder to add.

3. Deselect all the LESS files by ticking, then unticking the tick box up in the column header.

4. Select moodle.less from the list by ticking the checkbox beside it.

5. Right click on the moodle.less row and click Select output file.

6. Browse to theme\bootstrap\style\ and select generated.css as the output file.

7. Click the Compile button.

8. Swap to the Compiler tab to check the result of the process.