feat: apply dark/light colour palette to all ui elements

Author: tymzd

gcp/website/frontend3/img/background-1-light.png

@@ -48,7 +48,9 @@
     <header class="top-bar">
       <div class="logo">
         <a href="{{ url_for('frontend_handlers.index') }}" aria-label="Home page">
-          <img alt="OSV logo" src="/static/img/logo.png" srcset="/static/img/logo.png, /static/img/logo@2x.png 2x"
+          <img alt="OSV logo" class="logo-dark-mode" src="/static/img/logo.png" srcset="/static/img/logo.png, /static/img/logo@2x.png 2x"
+            width="54" height="20">
+          <img alt="OSV logo" class="logo-light-mode" src="/static/img/logo-dark.png" srcset="/static/img/logo-dark.png, /static/img/logo-dark@2x.png 2x"
             width="54" height="20">
         </a>
       </div>