In this tutorial you’ll find step-by-step instructions on how to add a custom CSS file in Magento 2.
1. Open the app/design/frontend/Zemez/themeXXX/Magento_Theme/layout/default_head_blocks.xml file and insert the following code before the </head> tag.
2. The next step is to create custom.css file with your custom CSS styles and overrides in the app/design/frontend/Zemez/themeXXX/web/css folder.
3. Remember to clear magento cache.
Useful Tutorials:
Cascading style sheets (CSS) in Magento
You also can add custom CSS style via Admin Panel.
Magento 2. How to Add CSS Styles to Magento Theme
