most usually it's the theme is lacking a
style.css file (or is corrupted or it doesn't have the right permissions). Check that out to make sure that is not the case. Mobile plugins could be causing this as well, like Fris said.
Anyway, the simple, easy solution is what baddog said: name your theme as
default. Problem with this is every single time you upgrade WP, you'll replace that default theme, so you better have a backup copy for the theme
So this is what I would do: open your wp-includes/theme.php file. Look for this:
Code:
function validate_current_theme() {
// Don't validate during an install/upgrade.
if ( defined('WP_INSTALLING') || !apply_filters( 'validate_current_theme', true ) )
return true;
if ( get_template() != WP_DEFAULT_THEME && !file_exists(get_template_directory() . '/index.php') ) {
switch_theme( WP_DEFAULT_THEME, WP_DEFAULT_THEME );
return false;
}
if ( get_stylesheet() != WP_DEFAULT_THEME && !file_exists(get_template_directory() . '/style.css') ) {
switch_theme( WP_DEFAULT_THEME, WP_DEFAULT_THEME );
return false;
}
return true;
}
and get rid of it.
Now a warning: there's a reason why that function is there. Use this workaround only if you can't figure what's going on with your theme. And again, every time you upgrade this file will be replaced, so keep a copy or edit the new upgraded theme.php file