PHP Notice: Undefined index: custom_css_url

Not using custom css.

For custom_css_url (introduced in 3.1.5), this is a minor issue since the default is '' (empty string) anyway. But any new option added by an update does not get defined before a Dashboard save, so maybe there are ones with non-empty default that would cause problems? Don’t know.

You could update-proof it by changing this:

// Retrieve the plugin options
$this->options = get_option( $this->option_name );

// If the options array is empty, set defaults
if ( ! is_array( $this->options ) )
  $this->set_defaults();

// If the version number doesn't match, upgrade
if ( $this->db_version > $this->options['db_version'] )
  $this->upgrade();

to

$this->set_defaults();

// Retrieve the plugin options
$opts = get_option( $this->option_name );

// Merge options
if ( is_array( $opts ) {
  $this->options = array_merge( $this->options, $opts );

// If the version number doesn't match, upgrade
if ( $this->db_version > $this->options['db_version'] )
  $this->upgrade();

Though there may be a problem with doing the upgrade() last.