Possibly you have an error in the path of the asset, maybe a folder, file or module name: 

...

<link href="/theme_name/static/src/less/customize.less" rel="stylesheet" type="text/less"/>
...

Theme name is different i'm mentioning the theme_xxxxx(name) for example.

Inheriting webiste.assets_editor for javascript is working fine but for defining css stylesheet its not working.

<template id="my_js" inherit_id="website.assets_editor" name="My Js">

<xpath expr="script[last()]" position="after">

<script type="text/javascript" src="/theme_name/static/js/theme_editor.js" />

</xpath>

</template>