To add the package, css_colors, to an app:

Issue the command while being inside the project directory
    
      flutter pub add css_colors

Import it
    
      Add a corresponding import statement in the Dart code.

Stop and restart the app, if necessary
    
      If the package brings platform-specific code
(Kotlin/Java for Android, Swift/Objective-C for iOS),
that code must be built into your app.
Hot reload and hot restart only update the Dart code,
so a full restart of the app might be required to avoid
errors like MissingPluginException when using the package.

Removing a package dependency to an app using flutter pub remove

To remove the package, css_colors, to an app:

Issue the command while being inside the project directory
    
      flutter pub remove css_colors

The Installing tab,
available on any package page on pub.dev,
is a handy reference for these steps.