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
sudo apt-get install npm node npm install recess -g
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).
2. On the command line, install recess like this:
npm install recess -g
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. You will need to examine what you have altered or written to make sure it is complete and the syntax is correct.
Download WinLess and install it.
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.