Easier theme changing

[Tsunami014]

Is your feature request related to a problem? No.

I love the different themes, but going into the settings to change the theme each time is annoying.

Describe the solution you'd like
So, I would like a feature where you could specify the theme for each document easily.

Describe alternatives you've considered
This could happen by either:

• Specifying the theme in a header to the document which would be great for making everyone who views the slides have the same theme (or if you had multiple slides and wanted each one to be a different theme)
• Specifying it in the slides preview panel which would just make it easier to switch theme which I think would be better

Both would be good, but do what you can :)

Thanks!