feat: add new background color

[coraliefeil]

User Story

As a brand manager of the Solid Design System, I would like to give more color choices for background colors, so that designers have more options to choose from, and more flexibility when creating new layouts.

Suggested Solution

Add #051530 / blue.800 to variables as background color.
Add new color variant to sd-container.

DoR

• Item has business value
• Item all subtasks have been estimated by the team
• Item is clear and well-defined
• Item dependencies have been identified

DoD

• All subtasks have been closed