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Unfortunately I have no idea where the theme came from, the site was built two years ago and not by me

Where do I get a clean copy from?

Thanks Steve for your quick response.
I’m not tech savvy and this is my first time trying to resolve an issue like this so bare with me please…

I enabled wp_debug and wp_debug_log and tried going to /wp-admin/ again, this time I got a different message instead of the HTTP 500 error:
“Notice: Constant SERVER_PATH already defined in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/functions.php on line 3
Parse error: syntax error, unexpected ‘new’ (T_NEW) in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/option-tree/includes/ot-meta-box-api.php on line 279”.

I tried deactivating the plugins with the help of the link you provided. I couldn’t get on to phpMyAdmin through cPanel, it required a login and trying my cPanel and WP login details wasn’t working so I’m not sure how to get into that.
I tried the second option too by renaming ‘plugins’ to ‘plugins.hold’ and then going to /wp-admin/plugins.php but it was still giving me the HTTP 500 error.

I’ve been using FileZilla and I found two error logs, in wp-admin and another in public_html folders. Both had a lot of repeating text but the latter was significantly larger.
The error log from wp-admin is just repeating these two errors;
“[20-Aug-2019 02:48:21 UTC] PHP Warning: Declaration of plugin_findreplace::addPluginContent($links, $file) should be compatible with mijnpress_plugin_framework::addPluginContent($filename, $links, $file, $config_url = NULL) in /home/artandclassicfra/public_html/wp-content/plugins/find-replace/find_replace.php on line 17
[20-Aug-2019 02:48:21 UTC] PHP Parse error: syntax error, unexpected ‘new’ (T_NEW) in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/option-tree/includes/ot-meta-box-api.php on line 279”.

And these are the last errors from from public_html;
“[20-Aug-2019 04:14:53 UTC] PHP Warning: Declaration of plugin_findreplace::addPluginContent($links, $file) should be compatible with mijnpress_plugin_framework::addPluginContent($filename, $links, $file, $config_url = NULL) in /home/artandclassicfra/public_html/wp-content/plugins/find-replace/find_replace.php on line 17
[20-Aug-2019 11:01:50 UTC] PHP Warning: A non-numeric value encountered in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/woocommerce/single-product/price.php on line 30
[21-Aug-2019 00:23:08 UTC] PHP Parse error: syntax error, unexpected ‘$customcsslist’ (T_VARIABLE) in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/archive.php on line 1
[24-Aug-2019 14:44:50 UTC] PHP Notice: Constant SERVER_PATH already defined in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/functions.php on line 3
[24-Aug-2019 14:44:51 UTC] PHP Notice: Constant SERVER_PATH already defined in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/functions.php on line 3
[24-Aug-2019 14:44:51 UTC] PHP Notice: Undefined index: background-color in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/includes/theme-functions.php on line 11
[24-Aug-2019 14:44:51 UTC] PHP Notice: Undefined index: background-image in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/includes/theme-functions.php on line 12
[24-Aug-2019 14:44:51 UTC] PHP Notice: Undefined index: background-repeat in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/includes/theme-functions.php on line 13
[24-Aug-2019 14:44:51 UTC] PHP Notice: Undefined index: background-attachment in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/includes/theme-functions.php on line 14
[24-Aug-2019 14:44:51 UTC] PHP Notice: Undefined index: background-position in /home/artandclassicfra/public_html/wp-content/themes/ArtandClassic/includes/theme-functions.php on line 15”.

Hope this helps to identify the issue, I am at a loss