You need to add your custom css file as a resource to your theme definition document. This way the custom css file gets loaded on the XPage.
Next, you add the attributes you want to change in your custom css file, e.g.:
.xspPanelViewColumnHeader {
color: green;
}
Depending on the order in which the CSS files are loaded, you might need to add the !important declration to the changed styles, e.g.:
.xspPanelViewColumnHeader {
color: green !important;
}
Good luck!
Mark