Card: Use raw values instead of CSS var

Created by: umpox

Description

Our CardX components relied on the card-spacer variable. This might work fine, but we don't consistently render our CardX components within a Card wrapper. This can create strange styling bugs where the spacer variables do not exist, or only exist in certain circumstances.

Test plan

1. Render a CardHeader outside of a Card component
2. Notice that CardHeader now has the correct padding