Fix theme handling and auto theme

This commit fixes the client side theme selection being overwritten by the backend and adds a selectable auto theming option.

ui/src/styles.sass

@@ -2,4 +2,10 @@
 
 /* Importing Bootstrap SCSS file. */
 @import 'node_modules/bootstrap/scss/bootstrap'
-@import '~@ng-select/ng-select/themes/default.theme.css'
+@import '~@ng-select/ng-select/themes/default.theme.css'
+
+.navbar
+    background-color: var(--bs-dark) !important
+
+    [data-bs-theme="dark"] &
+        background-color: var(--bs-dark-bg-subtle) !important