There are several ways to contribute to WordPress.
And the list goes on and on. Before I have mainly contributed by helping in support forums and building simple and accessible themes.
I shared my experiences in Post Status what was it like to contribute SVG icon system to Twenty Seventeen.
It was definitely educational process and hail for the open source.
If you want to jump right into code, all the main SVG functions can be found in inc/icon-functions.php file.
Or you can read the whole story about contributing to Twenty Seventeen in Post Status.
Big thanks to Leland Fiegel who helped me write the article.