Plans in Site Creation: Calypso UI improvements

[staskus]

More context: #82135

• When clicking the "Show all features" link of a plan, that link moves slightly up in the Jetpack App version, while in the .com plans one it stays on the same place: https://wordpress.com/pricing/
• A good idea for a follow-up: would be great to explore which styles we can more generically adopt for that component so they apply to all mobile devices. That could be a big win: in every other place where the FormattedHeader component gets utilized, it would already look well on mobile.
• Make fonts and colors to be aligned with the Jetpack app
• Add paid domain check (Task: Create Plans page for Jetpack App Site Creation #82135 (comment)) to load the default page if paid domain is not existent
• Dark mode

Source: #82135 (review)