according to my previous experience of changing element-ui, I need to add! important to overwrite the original style after the style I reset.

.ivu-tabs-nav-container{
  font-size:18px!important;
}

but in that case, all ivu-tabs-nav-container classes have a 18px font size

well, according to my previous experience of changing other UI framework styles, it's best to add a class to lock the element before you change it (such as the parent element) in case it changes to other elements.

.parent .ivu-tabs-nav-container{
  font-size: 18px;
}

in addition, if you are using vue, then if you want to change the style of the UI framework, do not change it in < style scoped > < / style > , but change < style > < / style > globally (because it is global, it is necessary to add one more class or something to limit its scope of action. )