Hi Shemeam,
It due to the custom CSS for EB in this file:
/media/com_eventbooking/assets/css/custom.css
.dropdown-toggle, .btn-default {
background-color: #07b !important;
color: #fff !important;
}
The classes like 'dropdown-toggle' 'btn-default' are often used in other parts of website also like the main menu so your custom rule will affect that parts.
My advice is that if you want to customize for element having these classes in EB, you should page suffix / module class suffix so it can be applied for that part specific.
Regards