If you're looking for syntax highlighting, one simple solution is to configure Aptana to recognize .less files as .css files.
To do this, navigate to the File Associations tab within the Aptana preferences window. You can easily find this by searching for 'File Associations' in the search bar located under General > Editors.
Within this section, you can create a new file type with the extension '*.less'. This will properly categorize the file and may even provide default associated editors.
To set up the association, click on Add... and select the 'CSS Source Editor' from the options provided. Once you click OK, it will be added to the list of editors - make sure to set it as the Default editor for .less files.
After following these steps, when you open a .less file in Aptana, you'll benefit from CSS syntax highlighting and autocomplete functionality.