Add aria-label to theme toggle button for accessibility (#1669)

Author: C-Loftus

layouts/partials/header.html

@@ -74,7 +74,7 @@
             {{- end }}
             <div class="logo-switches">
                 {{- if (not site.Params.disableThemeToggle) }}
-                <button id="theme-toggle" accesskey="t" title="(Alt + T)">
+                <button id="theme-toggle" accesskey="t" title="(Alt + T)" aria-label="Toggle theme">
                     <svg id="moon" xmlns="http://www.w3.org/2000/svg" width="24" height="18" viewBox="0 0 24 24"
                         fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round"
                         stroke-linejoin="round">