If you want to add custom CSS or JS to your website, you can define it for each post and page on?post options?and page option or add them globally by the customizer:
To add custom CSS or JS by customizer (To define globally):
- Go to your WordPress Admin Panel
- Click?Appearance on the Dashboard
- Then click?Customize
- Navigate?General > Custom CSS Code/Custom JS Code
Video Tutorial
To add custom CSS or JS in a single post:
- Go to your WordPress Admin Panel
- Navigate?Posts > Add New
- Find Post Options Meta Box, under the edit box
- Then click the last tab,?Advanced Setting
To add custom CSS or JS in a single page
- Go to your WordPress Admin Panel
- Navigate?Pages > Add New
- Find Page?Options?Meta Box, under the edit box
- Then click the last tab, Advanced Setting
