[BUG] CSS variables with default RGB color doesn't work

Photoshop
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1

None of these work:

background-color: var(--bg-color, rgba(0, 0, 0, .7));
background-color: var(--bg-color, rgb(0, 0, 0));
background-color: var(--bg-color, #00000000);

These work:

background-color: var(--bg-color);
background-color: var(--bg-color, #000000);