Highlight active selectize dropdown elements in dark mode

They are highlighted in light mode, and it makes sense to highlight them in
dark mode too.

.active is the hovered option, while .highlight is matching-user-input
letters in the option.

share/static/css/elevator-dark/main.css

@@ -457,3 +457,11 @@ body.darkmode.IE11 .ui-icon-circle-triangle-e {
 .darkmode th.collection-as-table svg.fa-sort * {
     color: #777 !important;
 }
+
+.darkmode .selectize-dropdown .highlight {
+    background-color: #7a7d8d !important;
+}
+
+.darkmode .selectize-dropdown .active {
+    background-color: #3a474d !important;
+}