How can I change the system theme in the Gnome edition?

Settings → Appearance → Style you can choose light or dark system theme.

Under Appearance you can change components of the theme like cursor or icons. Shell is greyed out until you install gnome-shell-extensions and enable User Themes in the Extensions applet. then you have the option to upload any theme you like.

The User Themes extension also has a couple built-in themes you can select from the settings window. They are lighter than the default theme, so check them out before you scour the internet looking for a new theme.