Changing design management buttons to tertiary and reducing margins

What does this MR do and why?

This MR addresses a couple small UI discrepancies in the Design Management overlay:

• Changes the action buttons in the top bar to be tertiary buttons (keeping pagination as-is)
• Modifies some of the classes being applied to the close button to align to our standard tertiary buttons (text color was previously off)

🎨 Designs seen here

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to an issue with a design added (or add a design to an issue)
2. Open the design in the overlay
3. Notice the action buttons (to left of pagination) are tertiary rather than primary.
4. Verify that the close button ("X" in top right) aligns to the other tertiary buttons.