Warn on unitless zero values in theme.json

[dsas]

What problem does this address?

When themes declare a global padding (styles.spacing.padding) in theme.json as 0 without a unit, it results in a display issue in the overlay menu on small screen resolutions.

0	0px

There's some work done in #53725 to better address padding, but it doesn't solve for the "unitless zero" case. This isn't something that can be solved easily, if at all, the calc css function is typed and so needs the type passing in.

Given the issue can't be addressed in code, it would be good to help theme authors be more aware of this problem

What is your proposed solution?

When theme.json is parsed - a warning/error could be emitted when a unit-less value is encountered in global padding (and possibly other places).

[ramonjd]

Font size discussion that could be related:

• Typography: numerical font size presets break FontSizePicker #44857 (comment)

Many value in theme.json are declared "string" according to the schema: https://github.com/WordPress/gutenberg/blob/trunk/schemas/json/theme.json#L564

I agree that a warning could work. Type coercion might trigger unexpected bugs.