MT Quartz Magento Theme Less CSS Precompiler Issue

Question

I'm using the MT Quartz magento theme. It uses the LESS css pre-compiler. The theme settings have an option to change the theme colours. These get written to a file on the file system design_default.less. There is another file at the same level named design_default.css. Changing the colours in the back end result in the less file being updated. However, the css file does not get updated when this happens. The frontend calls the css file for styling, which means that the front end colours do not update. Anyone has any idea why this could be happening? I'm new to css pre compilers and any help would be appreciated. Thanks much in advance.