GitLab UI integration branch for dark-mode-color-design-tokens

What does this MR do and why?

Integrates addition of $data-viz- and $theme- dark mode SCSS variables added as design tokens in GitLab UI in gitlab-ui!3818 (merged)

• Remove $theme-indigo-50 in _dark.scss inherits as #14143d
• Remove $indigo-900-alpha-008 and usage from redundant .contextual-sidebar
• Remove .marketing-popover as usage not found in GitLab

Screenshots or screen recordings

How to set up and validate locally

1. Review light/dark mode http://gdk.test:3000/-/profile/preferences
2. Ensure profile activity graph is correct in dark mode http://gdk.test:3000/root
3. Review CSS diffs diffs.zip

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.