A theme changes the global skin of Sublime Text 3. It changes the tabs, side bar, quick panels (goto, command palette), status bar, etc...
Here's a non-exhaustive list of themes (top 100):
Note: the convention for the theme packages name is that they start with
Theme - Focus img/ focus.light.sublime-theme focus.dark.sublime-theme
As you can see, this convention isn't really respected.
The themes are defined in a