Spacer: Add background color support

[hanneslsm]

There is a PR to this feature already, but it was closed without a proper discussion.
I strongly disagree with closing the PR. Therefore, this is a separate issue for the background color support for the spacer block. This feature is very much needed.

What problem does this address?

I often need an area with a background color but no content.
It's important that the size of this area can be defined with spacing sizes, because they use clamp.
The group block is not a viable solution, as outlined in my comment: #66433 (comment) The spacer is much better suited here, therefore, let's just add the design tools.

Also, a strong argument for adding the design tools is consistency in the editor, see the corresponding tracking issue: #43245

Other arguments for adding design tools to the spacer block were outlined in #43247 (comment) and #66443

What is your proposed solution?

Add the background-color design tool to the Spacer block, as in #66433

[aaronrobertshaw]

Paraphrasing some of the discussion over on #66433, here a few of the reasons we should hold off adopting block supports on the Spacer block.

• Semantic Clarity: The Spacer block is meant to represent negative space. Introducing visual styling shifts its purpose, potentially confusing its core meaning.
• Increased Complexity: Introducing additional styling options complicates the block’s API, potentially confusing consumers who expect a straightforward, purpose-specific tool. Moreover, this added complexity could undermine efforts to consolidate spacer functionality into the padding and margin styles of parent containers, helping to avoid excessive markup.
• Backward Compatibility Challenges: Once additional styling supports are introduced, there’s a long-term commitment to maintaining them. This can complicate future updates and add maintenance burdens.
• Alternative Solutions Exist: The desired visual effects can be achieved through other means—such as using a Group block —which maintains a clearer separation between layout structure and decorative elements.

[t-hamano]

I think it would be helpful to move this issue forward if we had more examples of what kind of design you want to achieve.

I expect that it might make more sense to extend the Separator block.

• In the Row block, change the separator vertically. Or provide an option to choose between vertical and horizontal layouts.
• Make the line thickness and width changeable.
• The Separator block supports not only hr elements but also div elements, so it can be used simply for visual purposes (Separator block: Allow divs to be used as separators #67530).

Perhaps a dedicated "Decoration block" would be good, but how people decorate will vary, and I feel that is the role of a plugin.