HTML

group by design bold vuetify table

<template v-slot:group.header="{ group, headers, toggle, isOpen }">
  <td :colspan="headers.length">
     <v-btn @click="toggle" x-small icon :ref="group">
         <v-icon v-if="isOpen">mdi-plus</v-icon>
         <v-icon v-else>mdi-minus</v-icon>
     </v-btn>
     <span class="mx-5 font-weight-bold">{{ group }}</span>
  </td>
</template>