Improve contrast of bit-descrs borders (#531)

Align the borders' colour with the header's.
High-contrast borders are important here, since those tables have a lot of `colspan`s,
and their primary purpose is to describe *layout*.

Author: vizigr0u

custom/style.css

@@ -177,6 +177,11 @@ mfrac msup :not(:first-child) {
     text-align: center;
 }
 
+/* Improve contrast on bit-descrs borders */
+.bit-descrs {
+  --table-border-color: var(--table-header-bg);
+}
+
 p, .table-wrapper {
   margin-block-start: 1em;
   margin-block-end: 1em;