A11y improvements and style tweaks

- Use button element instead of anchors, fixes keyboard navigation and
other issues
- Disable search input until it's open, prevents being able to write
inside it while it's closed
- Use :root instead of html in CSS
- Add minimal size for separator between home button and other links

templates/partials/search.html

@@ -191,7 +191,9 @@
 
     function toggleSearch() {
         var searchContainer = document.getElementById("search-container");
+        var searchBar = document.getElementById("search-bar");
         searchContainer.classList.toggle("active");
+        searchBar.toggleAttribute("disabled");
     }
 
     if (document.readyState === "complete" ||