Unable to set static front page

The option to select a static front page is not in Settings > Reading anymore, but in Appearance > Customize. When I select Static front page, two lines (“Front Page” and “Posts page”) appear below, but there is no dropdown to select the desired page.

So, it is impossible now to activate the knowlege base look as explained here: http://ipanelthemes.com/kb/wp-knowledge-base-theme/kb-components/activating-knowledge-base-appearance/

Any fix? Thanks.

(Tested in WP3.8 with Firefox and Chromium.)