Fix striped table backgrounds within blockquotes

Fixes #659 by defaulting to the primary background color for normal
table rows.  As noted in the issue, this still looks a bit weird,
but it's no longer inconsistent.

tildes/scss/_base.scss

@@ -280,6 +280,12 @@ table {
   margin-bottom: 1rem;
 }
 
+// Add a default non-transparent background to fix styling in blockquotes
+tbody tr,
+thead tr {
+  background-color: var(--background-primary-color);
+}
+
 tbody tr:nth-of-type(2n + 1) {
   background-color: var(--background-secondary-color);
 }