Unused classes in _uswds-theme-custom-styles.scss

[mahoneycm]

Description

As @mejiaj pointed out in #1573, there are classes mentioned in _uswds-theme-custom-styles.scss that are not found anywhere in the markup.

After looking through some more of the classes, it appears there are a lot of unused classes throughout the file.

Examples include:

• Preview box styles
• Button wrappers
• usa-color tokens

Steps to reproduce the issue

1. Navigate to css/_uswds-theme-custom-styles.scss.
2. Use cmd/ctrl + shift + f to search for various classes.
3. Check for any mardown or html files with reference to the classes exist.