Magento 2 After Create a Demo Theme CSS & JS not Load

First delete the current static theme files, if any, in order to apply newly added changes:

• You may already be familiar with developer mode. If you add the following to your .htaccess file ‘SetEnv MAGE_MODE developer’
• Remove all  files pub/static/frontend files of theme like /<myvender>/<mytheme>
• Remove all files var/view_preprocessed/source/frontend/<myvender>/<mytheme>
• Now clear var/cache and then check.