[themes] provide command to change theme

There is a command to open the theme prompt (`workbench.action.selectTheme`), however, I would like a command to change the theme (the name of which would be provided as an argument). The reason I would like this is that I frequently switch between a dark and a light theme, depending on my mood, time of day, etc. Currently, I am forced to use the theme picker, but I would really like to create keyboard shortcuts to switch themes immediately.