If you want to customize CSS only then you can use Additional CSS in Customizer. You should NEVER modify theme or plugin directly. Otherwise it will be lost when theme is updated. Another alternative is using child theme. https://codex.wordpress.org/Child_Themes
Regarding logo, CSS you have changed is correct. But please keep it in Additional CSS box in the Customizer.
Moreover, we are releasing theme update with this logo issue and other. Please check after you have updated theme.