Add Background option for all view

[pedro2d10]

Description:
Currently, in Home Assistant dashboards, background images can only be set per view. I would like to request a feature that allows users to define a global background image that applies to all views within a dashboard. This would improve visual consistency and simplify dashboard theming.

Proposed Solution:

Add an option to set a global background image at the dashboard level (not just per view). And then add in yaml before the view option the background field.

Benefits:

Provides a consistent look and feel across all views.

Reduces the need to manually set the same background on each view.

Enhances usability for users designing themed dashboards.

Thanks for considering this request!