You only need to call updateSettings once to update both settings. To avoid having a ridiculously long line of code, just do something like this:
```php
$known = strtr($modSettings['knownThemes'] . ',' . $id_theme, array(',,' => ','));
$enable = strtr($modSettings['enableThemes'] . ',' . $id_theme, array(',,' => ','));
updateSettings(array('knownThemes' => $theme_list, 'enableThemes' => $theme_list));
```