Currently themes are not applied to error messages (maintenance message is treated like "System Temporary Unavailable" error). You can change error.css in /public/assets/stylesheets folder or change layout to include any details you want. Layout that is used is defined in error.tpl file in /activecollab/application/modules/system/layouts folder.
We'll see to add theming support to error pages as well in one of the upcoming releases.
Is there a way to modify either the theme css or maintenance page to match the theme?