Gnome extensions are add-ons that can be added to modify and enhance system shell functionality. These tools can vary from modifying how the dash and panel behave, to forcing the display to "stay awake".
Gnome themes are add-ons that modify the look and color scheme of windows.
There are a few tools you can download from you distos Software store to customize Gnome:
- Gnome Extensions - released as of Gnome 3.36, it is limited to enabling and configuring settings for extensions
- Gnome Tweaks - an application for tweaking system settings, adding extensions, and changing system fonts and behavior, and changing system themes
- dconf Editor - a tool that exposes the backend configuration for editing system settings; it is a complex tool and not recommended to new users
Here is a walk-through video of how to customize Gnome: