static function getBaseTheme( $slug ) {
		$filename = static::getCustomCssFolder() . sanitize_file_name( $slug . '.css' );
		if ( ! file_exists( $filename ) ) {
			return false;
		}
		$theme = get_file_data(
			$filename, [
				'ThemeURI' => 'Theme URI',
			]
		);
		$theme_slug = str_replace( [ 'http://pressbooks.com/themes/', 'https://pressbooks.com/themes/' ], [ '', '' ], $theme['ThemeURI'] );

		return untrailingslashit( $theme_slug );
	}
static function getBaseTheme( $slug ) {
		$filename = static::getCustomCssFolder() . sanitize_file_name( $slug . '.css' );
		if ( ! file_exists( $filename ) ) {
			return false;
		}
		$theme = get_file_data(
			$filename, [
				'ThemeURI' => 'Theme URI',
			]
		);
		$theme_slug = str_replace( [ 'http://pressbooks.com/themes/', 'https://pressbooks.com/themes/' ], [ '', '' ], $theme['ThemeURI'] );

		return untrailingslashit( $theme_slug );
	}
static function getBaseTheme ( $ slug ) { $ filename = static :: getCustomCssFolder ( ) . sanitize_file_name ( $ slug . '.css' ) ; if ( ! file_exists ( $ filename ) ) { return false ; } $ theme = get_file_data ( $ filename , [ 'ThemeURI' => 'Theme URI' , ] ) ; $ theme_slug = str_replace ( [ 'http://pressbooks.com/themes/' , 'https://pressbooks.com/themes/' ] , [ '' , '' ] , $ theme [ 'ThemeURI' ] ) ; return untrailingslashit ( $ theme_slug ) ; }
Determine base theme that was used for the selected Custom CSS.

@param $slug string

@return string
Determine base theme that was used for the selected Custom CSS .