Tweak code CSS a bit to make it nicer

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

assets/scss/code.scss

@@ -55,6 +55,12 @@ pre code {
         .lntd:last-child {
             width: 100%;
         }
+
+        // Insert padding to match the distance of the line number to its
+        // border.
+        .line {
+            padding-left: $code-lineno-spacing;
+        }
     }
 
     .lntr {
@@ -63,16 +69,27 @@ pre code {
 
     .lnt {
         display: block;
-        padding: 0 1rem 0 1rem;
-        color: $code-lineno-color;
+        padding: 0 $code-lineno-spacing 0 $code-lineno-spacing;
+        @include var(color, code-lineno-color);
+        border-right: $standard-border;
+
+        // Even though only highlighted line table numbers get the 'star',
+        // insert a corresponding space on non-highlighted ones to keep
+        // spacing consistent.
+        &::before {
+            content: " ";
+        }
     }
 
     .hl {
         display: block;
         @include var(background-color, code-highlight-color);
 
-        .lnt::before {
-            content: "*";
+        .lnt {
+            border-left-color: $border-color;
+            &::before {
+                content: "*";
+            }
         }
     }
 }