Rename instances of "accent" color tokens with "highlight"

[slowbot]

• We have been using the names "accent-cool" and "accent-warm" for color tokens
• We want to update all these instances to use "highlight-cool" and "highlight-warm" across the codebase

[slowbot]

@jaredcwhite Do you want to add more clarity here?

[jaredcwhite]

@slowbot Sounds good to me!