This hasn’t been asked before, and isn’t currently possible. We’ve just been discussing the possibility of clicking the ‘MT’ in the top left to fully collapse both the left toolbar and the options at the top. Clicking it again would expand both sets of options. What do you think?
Also, have you considered switching to full screen mode using the F11 key in windows (firefox/chrome)
or Cntrl+Cmd+F on mac in firefox
or Cmd+Shift+F on mac in chrome
You can do this while editing in Microthemer too to see more of the site you’re editing. To exit full screen mode, you can normally move your mouse to the top of the screen, hit F11 again, or press the escape key.