My new landing page

David, that is already in the css?

Yes, edit that rule and add the box-sizing: border-box; property to it. Then you can remove the wildcard rule.

It works! Thanks!
David, to edit the css you only have a little bit of space.
Can you recommend a plugin to edit the css that works well with the theme?

Tom's Simple CSS:

https://wordpress.org/plugins/simple-css/

It provides a full code mirror in the Dashboard > Appearance.
Not much can be done about the Customizer editor without some serious set up.

You may want to download an IDE to your desktop where you can edit code more freely, they include syntax checkers etc. A simple one is brackets.io

Thank you David, I will check it out 👍

You're welcome

David, I have installed "Simple CSS", but do not see my own costum css.
Do I need to transfer my custom css to Simple css?

Yes :) You will see there is a Simple CSS in the Customiser as well so just cut and paste.
Note - Simple CSS also provides a Meta Box in the Page or Post editor. So if you need to, you can add page specific CSS

Awesome! It only states that the plugin has not been tested with the current Wordpress version. No problem?

not at all. :)

I am now working to make all css neat. What a space suddenly! hahaha! 😁👍

haha awesome