style: use --accent-underline on sponsor link underline

Swaps --line-strong for --accent-underline on the 'Become a sponsor'
text-decoration-color so the underline matches the tan hover underline
on project-name links like thealgorithms.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vinta

website/static/style.css

@@ -424,7 +424,7 @@ kbd {
   font-weight: 700;
   letter-spacing: 0.01em;
   text-decoration: underline;
-  text-decoration-color: var(--line-strong);
+  text-decoration-color: var(--accent-underline);
   text-underline-offset: 0.2em;
   transition:
     color 180ms ease,