Enable the dark mode on production

[Belco90]

What feature would you like to have?

The dark mode is enabled only for testing and preview purposes for now.

When new colors are setup properly (#243) and we switch to the new tokens by default (#244), we should be ready to put the correct button for switching the color scheme and enable it in production.

We can get rid of the NEXT_PUBLIC_FEATURE_FLAG_COLOR_MODE (including the section at CONTRIBUTING.md file) at the same time.

What problem will this feature solve?

Enables the dark mode in the production platform.

Do you want to help make the feature?

Yes

Is there anything else we need to know?

No response