Add makeshift image for dark mode

lib/outlook_web/components/layouts/public.html.heex

@@ -1,8 +1,10 @@
 <header class="">
   <.breakpoint_indicator :if={Mix.env == :dev} />
   <a href="/">
-    <img class="w-full" src="/images/elbefoto-lg.jpg"
+    <img class="w-full dark:hidden" src="/images/elbefoto-lg.jpg"
       src-set="elbefoto-xxl.jpg 4496w, /images/elbefoto-lg.jpg 2248w, /images/elbefoto-md.jpg 1199w, /images/elbefoto-sm.jpg 991w, /images/elbefoto-xs.jpg 767w">
+    <img class="w-full hidden dark:inline" src="/images/nadjas-nachtfoto-xxl.jpg"
+      src-set="nadjas-nachtfoto-xxl.jpg 1280w, /images/nadjas-nachtfoto-md.jpg 1199w, /images/nadjas-nachtfoto-sm.jpg 991w, /images/nadjas-nachtfoto-xs.jpg 767w">
   </a>
   <.dark_mode_widget />
 </header>