Install Code Customizer is a straight forward process like any normal Shopify apps:
Go to Shopify app page, and click Add app button. You will redirect to a page like this:
Click Install app button, and then Code Customizer app will be installed in your shop.
By default, app is disabled to prevent any effect on your shop. To enable app, go to Settings page and click Enable app button:
After that, Custom CSS will be available for enabling, click Enable custom css button to enable Custom CSS:
Go to Custom CSS page, in the editor, you can add your custom CSS code:
Click Save button to save and apply your CSS code to your store immediately.
Go to your Shopify store and see CSS applied to your theme.