Switches between one set of syntax and UI themes and another when you hit a key or visit its menu item.
Ideal for themes like Solarized that provide both light and dark versions that you may want to switch to under different lighting conditions. You'll want to pick a UI theme that looks good with your syntax theme. The defaults aren't bad, though.
You can override the defaults in the package settings. Use the package names of the syntax and UI themes (ex: unity-ui).
Keymapped to F12 by default.