_extend.less not working

I created a _extend.less under the following directory

web–>css–>source–>_extend.less

and added a simple .full-width{width:100%} then went to a phtml file and added the full-width class to an h1 element but itš not working. I see in dev tools that the h1 element has the class but no styling is associated to it.

For info i am in a custom theme that extends luma theme