Uninstalling styles and Default Style settings
If you uninstall themes in ACP XS styles, it checks that the selected theme to uninstall is not set to board config, or else display an error message. The relevant lines are in adm/xs_uninstall.php:
$remove_id = intval($_GET['remove']);
if($board_config['default_style'] == $remove_id)
{
xs_error(str_replace('{URL}', append_sid('xs_styles.' . $phpEx), $lang['xs_uninstall_default']) . '<br /><br />' . $lang['xs_uninstall_back']);
}
If an error message is not displayed, then there is a possible problem with xs_error() not being included, however, I cannot see why this is the case - the function is defined in xs_include.php, and that file is included at the top of xs_uninstall.php, and more to the point, if that function didn't exist, then you would get an error for a call to an undefined function.
Another possibility might be suggested that the variables are not correctly parsed to the function, but if that was the case, a blank error message would be displayed, so there would still be something to stop you removing a theme defined as the site default - so far this has always worked for me.
However, if a theme is removed by deleting it from the themes directory, or else renaming the folder, then the xs styles uninstall script will not have had any chance of being executed, and thus it would be possible to remove a theme that was defined as a default style. In this final scenario, the bug is definitely the user ;)