Use correct color for tabs border (#412)

Author: jarrodmillman

assets/theme-css/tabs.css

@@ -46,8 +46,8 @@
 
 /* The tab panel is in which the tab contents appear */
 [role="tabpanel"] {
-  border-top: 1px solid var(--pst-color-text-muted);
-  border-bottom: 1px solid var(--pst-color-text-muted);
+  border-top: 1px solid var(--pst-color-border);
+  border-bottom: 1px solid var(--pst-color-border);
   margin-top: 1px;
   padding-top: 0.5rem;
   padding-bottom: 0.5rem;