Congrats for that release! How can I activate the flat theme ?
Go to System Preferences, open "Screens", double-click on any open screen and choose a desired window skin from the list. (You can a choose a different window skin for each open screen if you like.)
Next, go to MUI Preferences and click on the "Settings" button in the upper left corner. This will open a menu that allows to choose between various bundled (or user-generated) theme presets. Make sure to save your changes by clicking the appropriate button.