The simplest way to add spacing is by using SLDS CSS classes:
<h3 slot ="title" class="slds-card__header-title slds-media__body">
<lightning-icon icon-name="standard:channel_program_history" size="small" ></lightning-icon>
<strong class="slds-m-left--x-small">RELISTED OPPORTUNITY</strong>
</h3>
or custom CSS classes:
<h3 slot ="title" class="slds-card__header-title slds-media__body">
<lightning-icon icon-name="standard:channel_program_history" size="small" ></lightning-icon>
<strong class="custom-class">RELISTED OPPORTUNITY</strong>
</h3>
/* lwc.css */
.custom-class {
margin-left: 4px;
}
<strong style="padding-left: 10px">RELISTED OPPORTUNITY</strong>
– Arnold Jr. Oct 30 '19 at 18:52