I first tired to double click the certificate, but the import option was grayed out.
I have looked through the archwiki and I found this article on how to add a custom certificate
certutil -d "sql:$HOME/.pki/nssdb" -A -i rootCA.pem -n "My CA" -t C,,
I’ve looked through the man page for certutil but I can’t seem to find a way to list all installed certificates to verify whether it worked or not.
I was following this tutorial to make the certificate.
I unfortunately have very little knowledge about how SSL works, so I might be going in the completely wrong direction. I just need to create a SSL certificate that the OS trusts that’s pointing to localhost (preferrably only for dev-mode chrome), so that I can do my development with SSL enabled 
(webpack / express development)
ahh thats cool!
unfortunately am working with node / webpack.
But I did find a way to add the certificates globally in KDE → SSL preferences
Still don’t know how to check the nssdb / get the certificates to work actually 
(Am now at the stage where localhost uses an unsupported protocol. ERR_SSL_VERSION_OR_CIPHER_MISMATCH are haunting me x) )
Instead of globally trusting your certificate, I would use a new browser profile and import the certificate there.
Your other error is that there is a mismatch between the protocol and the port (http://localhost:443 vs https://localhost:80).
Or you have created an outdated TLS certificate (yes, we’re saying TLS nowsince 1999, not SSL). That tutorial is from 2018 and therefore is using deprecated and unsafe default.
I don’t know the correct values, but you should be able to find another tutorial with current security assumptions.