resolveThemeAgreementFile (filepath) {
    const current = path.resolve(this.themeAPI.theme.path, filepath)
    if (fs.existsSync(current)) {
      return current
    }
    if (this.themeAPI.existsParentTheme) {
      const parent = path.resolve(this.themeAPI.parentTheme.path, filepath)
      if (fs.existsSync(parent)) {
        return parent
      }
    }
  }
Resolve the absolute path of a theme-level agreement file,
return `undefined` when it doesn't exists.

@param {string} filepath
@returns {string|undefined}