Fix: (.spec-meta) Background for dark-mode (#495)

This fixes the appearance of the .spec-meta class's background color
in dark mode.

This should be merged when the theme submodule is updated to include
<scientific-python/scientific-python-hugo-theme#233>.

assets/css/custom.css

@@ -61,7 +61,7 @@ table.spec__table thead {
 }
 
 .spec-meta {
-  background: #eee;
+  background: var(--colorBlockBackground);
   padding: 0.5rem;
   display: inline-block;
   margin-top: 0.5rem;