The BlockNote macro placeholder has insufficient color contrast

Steps to reproduce

1. Edit a page containing a macro that has no output yet in the BlockNote editor, e.g. join an existing realtime editing session on a page with a velocity macro.
2. Look at the macro placeholder, e.g. macro:velocity.

Expected

The placeholder text has a contrast ratio of at least 4.5:1.

Actual

The text is #cfcfcf on #ffffff, a 1.55:1 ratio. The color comes from --bn-colors-side-menu in the .xwiki-macro-placeholder rule of blocknote-react/src/blocknote/macro.css.

Context

Found by the WCAG validation of CollaborationIT#restrictScriptMacroExecution, on the .xwiki-macro-block selector, starting with build 1420 of the XWiki Environment Tests. The placeholder style comes from XWIKI-24157. The check started failing when XWIKI-24374 made users who join a realtime session see placeholders instead of the macro output.