The last part of theme variables can't use letters? Only support number?

[ZCY-G]

I try to defined this variables in @theme{}, but only --color-neutral-66 and --color-neutral-99 work.
This is Tailwind's rule or CSS Variables rule?

  --color-neutral-1B: #1b1b1b;
  --color-neutral-66: #666666;
  --color-neutral-99: #999999;
  --color-neutral-BB: #bbbbbb;
  --color-neutral-EE: #eeeeee;
  --color-neutral-F8: #f8f8f8;
  --color-neutral-FF: #ffffff;

[ZCY-G]

PS: The prettier extend in VSCode can work with this all variables.

[wongjn]

They seem to work fine in this Tailwind Play, so it could be something particular to your project stopping them from working.