fix(style): improve gist markdown file rendering

- Background now correctly uses the theme default, allowing render of markdown to be seamless.
- TODO: Nested code highlights from the markdown file itself still don't render with theming correctly

assets/css/_partial/_single/_code.scss

@@ -418,3 +418,21 @@ pre,
     color: $global-font-color-dark !important;
   }
 }
+.markdown-body {
+  background-color: $global-background-color;
+  color: $global-font-color;
+
+  .highlight {
+    background-color: $code-background-color;
+    color: $code-color;
+
+    [theme="dark"] & {
+      background-color: $code-background-color-dark;
+      color: $code-color-dark;
+    }
+  }
+  [theme="dark"] & {
+    background-color: $global-background-color-dark !important;
+    color: $global-font-color-dark !important;
+  }
+}