Hi Ciaran,
I tried to change the appearance of the content header and the module header for the following project:

https://www.knechtwebtestet.de/basic_4

The module header works showing the underline of the module header by using the following code in the custom.css:

.module_header h3 {
border-bottom: 2px solid rgba(255,183,0,1.0);
margin-bottom: 4px;
}

Now I tried the same with the content header:

.content_header h2 {
border-bottom: 2px solid rgba(255,183,0,1.0);
margin-bottom: 4px;
}

but this without sucess!!

Instead of content_header I also tried site-title but this also failed.

Any idea???

Jürgen