This is a known issue — will update you when I have more info.
I’m excited to let everyone know that a new build that saves the theme setting is rolling out this week! I’ll mark completed once it’s fully rolled out.
Thanks for your patience!Ed commented
I have never had a problem with losing this setting when using the 'Quit' option.
However if you click 'Sign Out' or 'Log Out' the theme preference is lost.