As mentioned in this forum post https://moodle.org/mod/forum/discuss.php?d=219045#p953994 a German Moodler has added the block version 2012120700 to Moodle 2.3.3+ and when clicking to book a room it appears as though the background theming has gone. I installed the block on my own localhost 2.3 from last week and see similar in English.