RSJommla Support Team says that is the JF Calla Templates that doesn't load the Bootstrap 2.3.2:
"I have taken a look and it appears that this issue is caused by the template you're using which does not load Bootstrap. RSMembership! as well as our other extensions have been designed by following the Joomla! standard which is Bootstrap 2.3.2 and in order the component to function properly and for best visual effects we recommend using Bootstrap 2.3.2. As you can see when loading the page using a default Joomla! template like Protostar, the memberships are displayed properly:
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