'Core' file won't open through FileZilla

I'm trying to do some theming (change logo, colors, etc) through filezilla. I'm using Vultr and was trying FileZilla to connect to the server but can't really open the 'core' folder, which to my understanding is where all the theming files are stored. Please correct me if I'm wrong. The error it gives me is: "Error: Critical file transfer error". I'm I doing something wrong? Also, why does the core file looks like a direct access link while all the most of the other folders look normal?

Thanks a lot!

I wonder how a malfunction of a 3rdparty tool is related to ownCloud? At the end owncloud is just PHP code which (from my knowledge) can't cause such an error.

I guess you're right. I'm new at this so it must have been a mistake Im making...