When dark theme is enabled at NC it is not possible to see which fulltext search provider to be selected.
Please, fix that in file js/admin.settings.js by commenting 147 and 148 rows:
146 tagSettingsAsSaved: function (div) {
147 // div.find('INPUT').animate({'backgroundColor': 'rgba(255, 255, 255, 0.18)'}, 300);
148 // div.find('SELECT').animate({'backgroundColor': '#fff'}, 300);
149
150 fts_admin_settings.updateEnabledSubProviders();
151 },
It will get rid of excessive animation which forcibly adds the white background.