diff --git a/theme/boost/templates/core_admin/setting_configduration.mustache b/theme/boost/templates/core_admin/setting_configduration.mustache
index 50c355d267f..c74645d2ec5 100644
--- a/theme/boost/templates/core_admin/setting_configduration.mustache
+++ b/theme/boost/templates/core_admin/setting_configduration.mustache
@@ -21,7 +21,7 @@
     <div class="form-inline">
         <input type="text" size="5" id="{{id}}v" name="{{name}}[v]" value="{{value}}" class="form-control text-ltr">
         <label class="sr-only" for="{{id}}u">{{#str}}durationunits, admin{{/str}}</label>
-        <select id="{{id}}u" name="{{name}}[u]" class="form-control">
+        <select id="{{id}}u" name="{{name}}[u]" class="form-control custom-select">
             {{#options}}
                 <option value="{{value}}" {{#selected}}selected{{/selected}}>{{name}}</option>
             {{/options}}