DayPilot default css styles

You can generate a custom theme using the Theme Designer:
https://themes.daypilot.org/

The default settings will generate a theme that corresponds to the built-in theme. You can include the generated CSS file using a <link> tag.

We use already the custom theme. As described above, we get error during registration of the default (built-in) CSS styles. Is it possible to disable that?

You can disable it by setting the value of DayPilot.Global.defaultCss to true (before you load DayPilot JS):

var DayPilot = {
  Global: {
    defaultCss: true
  }
};