Unfortunately, the only way is to use (or edit) the caption box CSS styling. The theme’s styling is done from this:
.banner-caption-content {
z-index: 2;
position: absolute;
top: 50%;
left: 0;
right: 0;
margin: 0 auto;
background-color: rgba(255,255,255,0.90);
text-align: center;
-webkit-transform: translateY(-50%);
-ms-transform: translateY(-50%);
transform: translateY(-50%);
}
.banner-caption-content:after {
z-index: -1;
position: absolute;
top: -10px;
left: -10px;
width: calc(100% - -18px);
height: calc(100% - -18px);
border: 1px solid #fff;
content: "";
}
.banner-caption-content-title {
margin-bottom: 0;
text-transform: uppercase;
font-size: 140%;
line-height: 26px;
letter-spacing: 6px;
}
.banner-caption-content-subtitle {
margin-top: 6px;
text-transform: uppercase;
font-size: 90%;
letter-spacing: 4px;
}
If you need to make changes to the CSS, and on WordPress 4.7, you can add your custom changes to the Additional CSS tab in the customizer, but to be honest, I would recommend a plugin called “Simple Custom CSS”. This gives you more working space and easy to manage. However, if you use Jetpack, it has its own Custom CSS feature you can enable instead, so you can also use that.
What was it you were wanting to change?