.item_price {
  text-align: left;
  line-height: 1.4;
  font-size: 14px;
  margin-top: 5px;
}

.item_price .regular {
  color: #888;
  text-decoration: line-through;
  display: inline-block;
}

.item_price .sale {
  color: #c00;
  font-weight: bold;
  display: inline-block;
}

/* ブランドリストはそのまま */
.top_brand_list_logo {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  margin: 0 auto;
}
.top_brand_list_logo li {
  width: 16.66%;
  margin: 0 1% 30px;
  list-style: none;
}
@media (max-width: 768px) {
  .top_brand_list_logo li {
    width: 15.66%;
  }
}
@media (max-width: 600px) {
  .top_brand_list_logo li {
    width: 22.66%;
  }
}

<style>
/* 強制上書き */
.item_price{ text-align:left; line-height:1.4; font-size:14px; margin-top:6px !important; }
.item_price .regular{ color:#888; text-decoration:line-through !important; display:inline-block; }
.item_price .sale{ color:#c00 !important; font-weight:700; display:inline-block; }
/* 旧クラスで出ていても赤/打消しに寄せる */
.item_price--regular{ color:#888 !important; text-decoration:line-through !important; }
.item_price--sale{ color:#c00 !important; font-weight:700 !important; }
</style>