use dimmed background for table headers

Currently all even table rows are dimmed down to help the reader's
orientation. The problem is that <tr> elements within a table are
further split into sections of <thead> and <tbody>. As a result, the
counter is not the some for all <tr> in the table, but there are two
separate counters for elements within <thead> and <tbody> respectively.

This can be visually distracting since the table appears to start with 2
light rows and only then the pattern of alternating backgrounds begins.

For most cases dimming the table headers works fine to help with this
issue.

Fixes: #23

assets/_custom.scss

@@ -36,3 +36,7 @@
 summary:focus {
 	outline: none;
 }
+
+.markdown table th {
+	background: var(--gray-100);
+}