Add a settings menu that can handle multiple themes

[thomasvardy]

I like to swap my themes fairly often and its inconvenient to modify the config file each time with my stored emails. I think a settings menu that could let you cycle through your themes would be a simple way to make changing your theme super easy.

I think that if there was support for a themes folder within the config folder then the config would just need a option for the theme name. This would allow you have multiple themes stored in one place and change then on the fly.

If there was a new shortcut added to launch the settings menu to select the theme then the settings menu could be expanded in the future for more customization/options

[isturiz]

I would really appreciate it.