Hi Dan - just assign css to whatever tag you wrap the title html in - I believe the default is the <b> tag.
So lets say your registration page is contained in the div bl_registration, set your css like this:
#bl_registration b {
background: red;
}
or you could assign a specific class to each title, ex. <b class="reg-title">Login Details </b> and set the css like this:
.reg-title {
background: red;
}
You'll probably want to add some padding so the background is larger than the title itself:
.reg-title {
background: red;
padding: 6px;
}