Implement new Cart and Checkout designs to improve theme compatibility

[Aljullu]

This was requested as an idea in:
https://ideas.woocommerce.com/forums/133476-woocommerce/suggestions/40549039-allow-for-themes-colour-scheme-to-be-used-in-the-b

We want to update the Cart and Checkout blocks so they play better with themes. Currently, most of their colors are hardcoded, but we would like to use the ones that we inherit from the theme. These are example designs from @LevinMedia:

Screenshots

Light theme

Cart	Checkout

Dark theme

Cart	Checkout

Issues & PRs

• Cart & Checkout: update item count badge so it works in light and dark backgrounds Cart & Checkout: update item count badge so it works in light and dark backgrounds #2617
• Update Cart & Checkout heading styles Update Cart & Checkout heading styles #2597
• Replace all hardcoded font-color declarations with alternatives Replace all hardcoded font-color declarations with alternatives in the Cart and Checkout blocks #2459
• Checkout: update progress indicator design Checkout: update progress indicator design #2618
• Cart & Checkout: style button depending on theme colors Cart & Checkout: style button depending on theme colors #2761 & Add Cart and Checkout block styles storefront#1412

[Aljullu]

Closing this issue since all its dependencies have been already merged.