Here is the Complete step by step article to adding JS and CSS as well as to managing in JS and CSS in Prestashop 1.7 versions.
Step1: Go to Your Classic/Specific Theme
Step2: Modify Your Theme
Go to Prestahsop Instllation Directory >> themes >>> Config >> theme.yml
Here is the structure How to call css and js file.
Step3: After that you want to reload the themes
You want to go to prestashop admin panel >>> Design >>Theme and logo >> Select updated theme.
Now Clear the cache
Congratulations! Your custom css is added in particular theme.