I am trying to change css of
Magento 2.1 Luma theme but its not working.
What will be the better way to modify css in
Magento 2.1 Luma theme ?
Login in admin => Stores => Configuration => Advance => Developer
In Front-end development workflow section Select Workflow type => Client Side Less Compilation and save config after that clean cache and then edit and check your changes.
You can also use grunt for less compilation check below links.