The second way is more flexible and accurate. It is preferable to create a new theme manually by Sass because the theme is better maintainable. The main CSS settings, such as font, colors, border radius, and many more can be done configurable by Sass variables. You can create a new theme by setting custom values for those variables. This is exactly the approach followed by PrimeNG. The mostly free themes were created in such manner.
Every theme has a separate folder with a Sass file where variables are set. The variables themselves are used in _theme.scss--shared file for all free themes. This file can be found under node_modules/primeng/resources/themes/, if you install PrimeNG as the dependency. Sometimes, you also need to set custom fonts or special settings for particular CSS selectors. You can overwrite default style rules with your own ones--just write them...
