Fix markdown table row background and borders. (#8553)

Author: isoos

pkg/web_css/lib/src/_base.scss

@@ -280,20 +280,20 @@ pre {
       border-top: none; /* overrides github-markdown.css */
 
       &:nth-child(2n) {
-        background-color: inherit; /* overrides github-markdown.css */
+        background-color: var(--pub-overlay-gray-08); /* overrides github-markdown.css */
       }
     }
 
     th {
       font-family: var(--pub-font-family-body);
       font-size: 16px;
       font-weight: 400; /* overrides github-markdown.css */
-      border-bottom: 1px solid #c8c8ca;
+      border-bottom: 1px solid var(--pub-overlay-gray-80);
       text-align: left;
     }
 
     td {
-      border-bottom: 1px solid #f5f5f7;
+      border-bottom: 1px solid var(--pub-overlay-gray-30);
     }
 
     img {

pkg/web_css/lib/src/_variables.scss

@@ -30,6 +30,13 @@
   --pub-color-white:         #ffffff;
   --pub-color-dangerRed:     #ff4242;
 
+  // These colors are a combination of middle-gray (#808080) and a transparency
+  // (alpha) channel, specifying an opaque color that mutes whatever the original
+  // color was behind it.
+  --pub-overlay-gray-08: #80808008;
+  --pub-overlay-gray-30: #80808030;
+  --pub-overlay-gray-80: #80808080;
+
   --pub-hash_link-text-color: #ccc;
   --pub-footer-background-color: #27323a;
   --pub-footer-text-color: #f8f9fa;