You can use these CSS codes as of our own support community:
.post-item .description {font-size: 14px}
.card-head-tag-wrapper i {font-size: 10px}
.card-head-group-name, .card-head-tag-wrapper a, .card-head-tag-wrapper span {font-size: 12px}
.question-title {font-size: 0.9em;}
.user-mention {font-size: 12px; font-weight: 500;padding: 0 3px;}
.user-bar {font-size:12px;}
.card-head-content-type {font-size:12px;}
.ui.card .meta, .ui.cards3>.card .meta {font-size: 12px}
.app-menu-item, .topic-list-item h3, .topic-menu-item {font-size:12px !important;}
.ui.divider {
font-size: 12px!important;
}
.ui.secondary.vertical.menu4>.item {
font-weight: 500;
}