Answered: how can find this code in the source code(which got from git hub)?

If you want to customize your "qa-nav-main-link" elements, you need to re-code the Q2A functions that output this.

Check these functions nav, nav_list, nav_item,  nav_link in qa-theme-base.php file. Also check out if some of them are already overridden in your qa-theme.php file.

Additional advice is that you must know that the core layout functions are in qa-theme-base.php file. If you are using some theme, like the default "Snowflat" theme, for customizing purpose, some functions can be overridden in qa-theme/Snowflat/qa-theme.php file.

You can apply your changes by 1) creating a plugin, by 2) editing the qa-theme.php file, or 3) by editing the qa-theme-base.php. The first option is advisable because your changes won't be affected by future updates.