Nishita's functions.php contains the following comment:
"To override nishita_setup() in a child theme, add your own nishita_setup to your child theme's functions.php file."
My initial act was to copy nishita's functions.php to the child theme's folder. This gave me errors related to two functions. Here's one of the errors. Commenting that function out reveals the other function's re-declaration error. After that there are two other errors related to the calls to the two commented-out functions:
Fatal error: Cannot redeclare nishita_calendar_widget_title() (previously declared in [child-theme's-path]/wp-content/themes/[child-theme]/functions.php:225) in [nishita-theme's-path]/wp-content/themes/nishita/functions.php on line 230
What is the practice to keep nishita's functions.php as-is (for incremental tweaking) and still avoid this error?