I can’t imagine what in the Ghost core would impact theme CSS though. Especially since 5.94.0 looks pretty harmless in itself.
FOSS does mean you can do what you want, yeah. You can choose not to update. Though that will leave you vulnerable to somebody literally fetching all data from your database, write new data into it, including putting malicious phishing and malware banners on your site through code injection:
You can do what you want – though I don’t see a logical argument in favour of staying on a known insecure version. The “external force”, in this case, is a known security vulnerability that we know is actively exploited. Not the Ghost Foundation or us telling you what to do.
The exact error trace would be a great start. What commands ran, what came back, etc.