Simon,
Here is the location and the rule to modify:
[install dir]server\apps\[app#]\themes\TabTheme\widgets\Header\css\style.css
.jimu-widget-header .jimu-title{
text-align: center;
height: 100%;
color: blue;
}
.jimu-widget-header .jimu-subtitle{
text-align: center;
height: 100%;
color: blue;
}