Check it out please -> FIDDLE
I downloaded the source from http://silviomoreto.github.io/bootstrap-select/, I really have no idea why it's not toggling "open" class of ".dropdown-toggle"
I can fix it by adding some messy scripts, but I prefer the plugin to work right.
<div class="number customdp">
<select class="simple-dropdown" name="number">
See this updated fiddle with the CSS and JS included, and it seems to work just fine.