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
When you add any code to functions.php, make sure, that you added it before closed tag ?>