Fix CTA button color on landing page

This commit fixes white-on-white text in the landing page's CTA button
when using light mode.

The background color of the button was removed in
3d9d8fb, but the white-on-white text
was prevented due to a typo (a missing semi-colon).  The typo was
subsequently fixed in fdac46e.

Author: jonathanhefner

docs/style.css

@@ -578,6 +578,7 @@ body:has(#schema-reference) {
     border-radius: 9999px;
 
     border: 6px solid var(--highlight-color);
+    background: #0e0e10;
     color: white;
 
     font-size: 1.5rem;