center and rework some utility screens

classes/handler/public.php

classes/opml.php

classes/pref/users.php

css/default.css

css/default.css.map

css/defines.less

css/tt-rss.less

css/utility.less

images/logo_wide.png

images/toolbar.png

include/sanity_check.php

install/index.php

register.php

themes/night.css

themes/night.css.map

themes/night_blue.css

themes/night_blue.css.map

update.php