Currently the only way is to edit the URL. When you click on a sound, the URL will end in something like ?q=a; change that a to two IPA characters and you can search for diphthongs.
(Also searching for a vowel will search for diphthongs including that vowel, so "ai" will be included in a search for "a".)
You mean the links (and the homepage background)? Because it's fall (in the northern hemisphere) and my website switches themes depending on the season.
(If you don't like it, you can change the theme here: https://chridd.nfshost.com/options/)