Updating secondary and reverse button to use correct variable rather the darken

[Fenwick17]

Description

When working on Figma @Asad-acc spotted that secondary and reverse buttons were using the SCSS darken to change their color on hover. We already have variables defined for this state, so this PR changes the buttons to use those variables.

Checklist

• [x] Tested against our testing policy (Resolution, Browser & Accessibility)
• [x] Follows our coding standards and style guide
• [x] CHANGELOG entry