@mathijx
I guess you have upload a wrong package when you install the theme, you should upload “oldcar_v.1.0.zip” which is in the package you downloaded, that’s theme zip file, do you understand? Please read the documentation, i have introduced how to install the theme in documentation.
Also, I have tested the logo replaced again, it works fine, so as @Musama said, after you upload the logo, please don’t forget click the “insert the post” button. If you clicked, but it still not display the logo, please post your wordpress login information here with private reply, I will check your setting.
Thanks!