> border-color: var(--vb-light) var(--vb-dark) var(--vb-dark) var(--vb-light);

This creates 45 degree corners on the borders which are an uncanny valley to me [1]. Feel free to lift any CSS you need from https://jdan.github.io/98.css/, it's MIT licensed. I used a box-shadow hack to get the "staircase" look.

> box-shadow: inset -1px -1px #0a0a0a,inset 1px 1px #fff,inset -2px -2px grey,inset 2px 2px #dfdfdf;

[1] https://notes.jordanscales.com/98-css-reflections#when-butto...