If a theme includes a `theme.json` file and sets a specific `font-style` on links, the font-style also changes the links in the WordPress admin bar, when the admin bar is visible on front-end. For example, this happens with the `pitch` style variation of Twenty Twenty-Three.
Using a default value for the `font-style` property prevents `theme.json` files from overriding it.
Props audrasjb, poena.
Fixes#56737.
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