Fix docs <code> elements having a black background (slint-ui#8453)

Fixes slint-ui#8331

Author: NigelBreslaw

docs/astro/src/styles/custom.css

@@ -53,7 +53,6 @@
     --sl-text-h4: 1.2rem;
     --sl-content-width: 50rem;
     --sl-color-bg: #1b1b1f;
-    --sl-color-gray-6: #161618;
     --sl-color-hairline: #212329;
     --sl-color-hairline-shade: var(--sl-color-hairline);
     --badge-border-color: --sl-color-hairline;