Right now, we can only customize background and foreground colors but no settings for alignment, etc.
At the moment there’s no settings for that. However there’s a pull request for this on GitHub that probably will be merged soon. Until then, if you know some CSS coding, you could manually edit the defaultpage.htm file in C:\Program Files\Quelea\server (for a Windows installation). If you want, you can look here at the PR for inspiration.