Replace all hardcoded font-color declarations with alternatives in the Cart and Checkout blocks

[Aljullu]

Hardcoded font-color make our blocks not to play well with different background colors: a black text will not be visible if the store has a black background. We should evaluate and implement alternatives depending on each case:

• Some times, two different font-weight values work as well as two different colors.
• Some other times, opacity is good enough to simulate different shades of the same color.

Some of this will need design approval/recommendations, so better discuss each change with designers.

[Aljullu]

Let's rename this issue so it's only about fixing this in the Cart and Checkout blocks. We have another issue to identify other blocks affected by this (#2539).