Documentation, where to modify general color, font face, template?

Hello,

I want to change the color of the links, the general color for the theme and the font.
Configuration of the news blog as the entrys overlay over the menu bar.
The button on top (creative button) to register, when I click on it there is an error page coming.
As I cannot find any documentation, please come back for some hints.
I cannot imagine that it was supposed to create a child theme to modify everything over the css file.

Thank you.
BR
Thomas