A bug has been reports on webtrees that prevents the drop-down menus for languages, favourites, and themes from functioning correctly. There are two fixes for this, a quick one for immediate use (attached here) and a more comprehensive one for the next release.
Attached here is a replacement for the file /includes/session.php. Just copy/replace that file with this one and the “colors” theme menus will work again as intended.
I have also prepared the code of that file, all the themes, and a couple of other related files to provide the same long term fix as is being applied to webtrees 1.5.1 (move ‘browser sniffing’ into themes where required). Those changes will replace this temporary copy of session.php at the next kiwitrees release.