Style nameing improvments.

assets/sass/base/document/_document.scss

@@ -12,6 +12,8 @@ html {
 body {
   // Correct the line height in all browsers.
   line-height: var(--base-line-height);
+  // Accent colour for forms.
+  accent-color: var(--color-accent);
   // Output a horizontal grid to help with debugging typography. The
   // $debug variable will toggle its output.
   @include debug-grid();