You can change the colours of the buttons on the event details page by adding the following CSS codes under Calendar Settings > Customize > Custom CSS box:

.timely-nav>li>a {
background-color: #d8e3e9 !important;
}

.timely-nav>li.active>a {
background-color: #ec008c !important;
}

.timely-nav>li>a:hover {
background-color: #9dbfd2 !important;
}

.timely-nav>li.active>a:hover {
background-color: #ec008c !important;
}