Hmm can you clear your cache? Your site is using minified css code and I think the old theme is still cached. Remove everything inside the following folders:

• /src/Frontend/Cache/CachedTemplates/
• /src/Frontend/Cache/Locale/
• /src/Frontend/Cache/MinifiedCss/
• /src/Frontend/Cache/MinifiedJs/
• /src/Frontend/Cache/Navigation/
• /src/Frontend/Cache/CompiledTemplates/

Ok done as instructed,

Still the same issue.

Also, when I click on Details on the Theme it says

"The Item Doesn't Exist"

Hmm I think it's because your theme is made for Fork CMS 3.6 and not 3.7. You can download Fork CMS 3.6.6 and the theme should work just fine. Fork CMS 3.6.6 and 3.7 are very similar (not really new features), but 3.7 uses Symfony's Dependency Injection and namespaces and because of that every folder and file is renamed to the CamelCased equivalent.

You can however modify the theme yourself to support 3.7. Then you have to take a look at the Triton theme folder and try to make Panakeia theme look similar regarding file structure. I think you must rename almost every folder to start with a capital letter/CamelCase name. And browse through the files to see if a file path has lowercase notation and change it to the right CamelCased notation.

In the meantime, I submitted a github issue for the Panakeia theme to update it to 3.7 asap.

Possibly might be good to add on each theme what version of ForkCMS they are compatible with.

Is there an easy way to downgrade?

There are many database changes in 3.7 so no, unfortunately there's no easy downgrade solution... Try a new install from Fork CMS 3.6.6.

And yes, many of the themes still need to be converted to Fork 3.7. The developer of Panakeia theme promised to convert it asap :)