Update dashboard styles to match latest designs

What does this MR do and why?

Updates the customizable dashboard styles to match latest designs from the new Create Dashboard flow

• Background is now white unless editing
• Panels always have solid border
• Panel title is muted

Note that product analytics is an experimental feature behind a default-off feature flag.

Screenshots or screen recordings

Before	After
	(viewing, also error state)
	(editing)
	(dark mode - default GL styles)

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard a project at Project -> Analyze -> Analytics dashboards
3. View a dashboard, and check styles are as expected

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #391699 (closed)