Fix: (styles.css) .navbar-menu colors (#407)

When the navbar menu is collapsed into the hamburger menu, the text
color had poor contrast against the background.  Now it's clearly
readable.

Author: alphapapa

assets/theme-css/styles.css

@@ -220,6 +220,12 @@ nav,
 .navbar-menu.is-active {
   background: var(--pst-color-text-muted);
 }
+.navbar-menu.is-active .navbar-item {
+    color: var(--pst-color-background);
+}
+.navbar-menu.is-active .navbar-item:hover {
+    color: var(--pst-color-link-hover);
+}
 
 a.navbar-item,
 a.navbar-item:visited {