Add navigation item highlight to active item

This change adds high lighting to navigation items in tech docs. It highlights
the whole nav tree with the same color and an underline.

Signed-off-by: Crevil <bjoern.soerensen@gmail.com>

.changeset/itchy-mice-kiss.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-techdocs': patch
+---
+
+Add highlight to active navigation item and navigation parents.

plugins/techdocs/src/reader/transformers/styles/rules/layout.ts

@@ -47,6 +47,11 @@ export default ({ theme, sidebar }: RuleOptions) => `
   height: 20px !important;
 }
 
+.md-nav__item--active > .md-nav__link, a.md-nav__link--active {
+  text-decoration: underline;
+  color: var(--md-typeset-a-color);
+}
+
 .md-main__inner {
   margin-top: 0;
 }