HTML Block: Add support for color and border

[scruffian]

What problem does this address?

Themes need a way to customize the HTML block, particularly the color and border properties.

What is your proposed solution?

We can add support for color and border to block so that it can be changed via theme.json.

[mtias]

Not sure I follow this one. The html block just renders the html on the front?

[scruffian]

In Quadrat for example we are modifying the color and border of the textarea in the editor, so that it matches the forms in the front end:

Without this the HTML block is pretty hard to use:

Perhaps this will be dealt with when we allow form elements in Global Styles...

[mtias]

This seems like the HTML block should force a white background. cc @jasmussen It's not meant to blend with the theme, it's a code view that will render the HTML on the front anyways.

[jasmussen]

Right, the HTML block should be considered "editor UI", and have the same materials and styling as the block toolbar. I'll see if I can increase the specificity of those styles to survive anything the theme throws at it.