The version number you listed, 11.0.2311.505585, is Forms 11 Update 5. I've been doing form headers with centered text and several other formatting changes since Forms 11 Update 3, so it can be done.
Here's the CSS I've been using:
/*Dictate the appearance of the section headers. They won't be as tall,
will be dark blue with rounded corders, and have white text.*/
fl-section .section-label {
text-align: center;
width: 100%;
background-color: #194077!important;
border-width: 0px!important;
border-radius: 15px;
padding: 0px;
margin: 0px;
color: #FFFFFF!important;
align-items: center;
justify-content: center;
line-height: 1.5;
font-size: 16px!important;
font-weight: 600!important;
}
fl-section .section-title {
border-bottom-style: none!important;
padding-right: 5px!important;
padding-left: 5px!important;
}
It results in section headers that look like this:
