I advise you to read more carefully the Magento fallback mechanism, about theme inheritance, in the official guides.
When inheriting, you don't have to necessarily create your own files, in the way you are writing your themes.js, if Magento doesn't find files in your custom theme, it will look automatically look for them in the parent theme.
You just need to write your file if you want to add some custom css, otherwise you can extend or override parent themes. Just look at the proper topic about extending or overriding a theme here
To stop grunt from watching your files, you have to manually first start it, otherwise it is not grunt that is compiling your files, but the built-in php pre-processor from Magento.
If you want to use grunt, first be sure you can run Grunt commands to compile and watch your theme, like this:
the last command takes no arguments and automatically watches for changes in themes folders specified in