Migrate inset colors (e.g. code and tab background)

[isoos]

• Partial reimplementation of New naming scheme for theme roles + migrating some background colors. #8064, only changing one color role at a time, doing with the inset background.
• Insets are items that are integral part of the regular page flow, like code blocks, tabs, even badges.
• color changes:
  • the light theme's simple badge's color goes from #f0f0f0 to #f5f5f7 (this is the background of the [unknown platforms] badge) - the dark theme already used the inset color as a background
  • the dark theme's color changes from #474747 to #41424c (anchor black) as one of the closest named color
  • fixed --pub-sort_control_hover-background-color as it should use the hover background instead of the inset background (planned in a subsequent PR)

[isoos]

Note: this has been pushed to staging.

[isoos]

Note: I think the new Flutter website is planning to use rgba(185, 174, 0, 0.2) as the dark theme code block background, but I found it way too bright, and rather keeping the current version for now. We may revisit this later.

[sigurdm]

Note: I think the new Flutter website is planning to use rgba(185, 174, 0, 0.2) as the dark theme code block background, but I found it way too bright, and rather keeping the current version for now. We may revisit this later.

cc @parlough