The Customizer is part of WordPress for quite a while now. If you still need basic instruction on how to use it, take a look here: wordpress.com/support/customizer/.
It’s a wordpress.com tutorial but pretty much works the same for the open-source version of WordPress.
After you entered the customizer you find a section called “Passster” click on it. The you should see a screen pretty similar to this:
You can now choose from three different sections to customise the look and feel of Passster: General, Form Instrctions and Button.
One note: The modifications here are global. That means the settings apply to all page protections and shortcode unless you overwrite them with the parameters shown in Shortcode.
Here you can edit the basic settings like the background color, padding, and margin. Passster uses the live mode, so any changes applied will be seen on the right side immediately.
Here you can modify everything related to the form itself. The description, error messages, the headline, font sizes, colors, and so on.
The last section controls the design of the button. You can modify the background and font color and also the hover appearance.