src/admin/class-papi-admin.php
public function admin_body_class( $classes ) {
		if ( $entry_type = $this->get_entry_type() ) {
			$classes .= sprintf( ' papi-body papi-meta-type-%s', papi_get_meta_type() );

			// Add custom css classes from entry type.
			$arr = $entry_type->get_body_classes();
			$arr = is_string( $arr ) ? [ $arr ] : $arr;
			$arr = is_array( $arr ) ? $arr : [];

			$classes .= ' ' . implode( ' ', $arr );
		}

		// Add custom css classes from query string.
		if ( $css = papi_get_qs( 'papi_css' ) ) {
			$css = is_array( $css ) ? $css : [];
			$css = array_map( 'sanitize_text_field', $css );
			$classes .= ' ' . implode( ' ', $css );
		}

		return $classes;
	}
public function admin_body_class( $classes ) {
		if ( $entry_type = $this->get_entry_type() ) {
			$classes .= sprintf( ' papi-body papi-meta-type-%s', papi_get_meta_type() );

			// Add custom css classes from entry type.
			$arr = $entry_type->get_body_classes();
			$arr = is_string( $arr ) ? [ $arr ] : $arr;
			$arr = is_array( $arr ) ? $arr : [];

			$classes .= ' ' . implode( ' ', $arr );
		}

		// Add custom css classes from query string.
		if ( $css = papi_get_qs( 'papi_css' ) ) {
			$css = is_array( $css ) ? $css : [];
			$css = array_map( 'sanitize_text_field', $css );
			$classes .= ' ' . implode( ' ', $css );
		}

		return $classes;
	}
Add custom body class when it's a page type.

@param  string $classes

@return string