Here is how you get those themes back:

Open the application menu(the button on the tool bar its the 3 horizontal lines)

Go under more tools and click Customize tool bar

On the bottom of the screen you will see 4 options one of which will be themes

click that and select the theme that you wish to have

Then you can customize further or close the tab

Hope this works, let me know if you need more help
-Lean_Beef