Low Search doesnt work, Free Form doesnt work, Free members - basically asks if your sure you want to login as some things might not be secure.
These add-ons usually use what you have configured in your "Admin > General Configuration > URL to the root directory of your site". You can also set/override the General Configuration settings using the /system/expressionengine/config/config.php file.
If using your config.php file to make the changes, you could use these basic settings to possibly fix some or all of the issues you're experiencing:
// check if https or http
$base_url = (!empty($_SERVER['HTTPS'])) ? 'https://' . $_SERVER['HTTP_HOST'] : 'http://' . $_SERVER['HTTP_HOST'];
// set as constant
define('BASE_URL', $base_url . '/');
$config['site_url'] = BASE_URL;
$config['cp_url'] = BASE_URL.'admin.php';
$config['theme_folder_path'] = BASE_PATH.'themes';
$config['theme_folder_url'] = BASE_URL.'themes/';
This would be due to the /themes/ folder path setting being invalid/incorrect. You can update it using the General Configuration or config.php file I mentioned above.
Do i need to change some files to reflect https:// for everything to work properly?
If you've configured your paths to be relative or based on the current URI, you shouldn't need to setup any special https redirect rules. However, I do recommend either forcing www or non-www for SEO and to prevent cross-domain errors. This would be done using your htaccess file: