If you need to make some small CSS changes – place them in Theme option General Custom CSS .
If you use Rehub and want to add deep customizations, use blank child theme which you can find in full theme bundle or download and install rehub-blankchild
If you need to overwrite core files of theme – just place them in child theme folder in the same way as they exist in main theme. So file
/inc/parts/query_type1.php must have the same folders inside plugin’s folder to overwrite file. You can overwrite only template files, but you can’t overwrite function files from folder function/. Instead of this, copy function which you want to overwrite and place it in functions.php file of child theme