Hi Shirley,
If we talk about CSS, in general we do not teach CSS here. I hope you read & know why.
In your case, for a single use, I would add only that HTML:
<h3 style="display: block; text-align: center">
<span style="font-size: 36pt; font-weight: 600; border-bottom: 15px solid #888;">
HOW WE DO IT</span>
</h3>
But delete that part
h3::before {
content: '';
position: absolute;
left: 0;
bottom: 0;
width: 100%;
height: 5px;
background-color: gray;
}
h3 {text-align: center !important; position: relative; display: inline-block;}
becuase in that form are NOT needed !
If this same H3 style with line you want to use on all subpages, syntax is also simple:
<h3><span>HOW WE DO IT</span><h3>
with basic CSS:
h3 { display: block; text-align: center;}
h3 > span { font-size: 36pt; font-weight: 600; border-bottom: 15px solid #888;}