Why is it that every time I put my mac to sleep it resets the dark menu bar?
I have an application called flux and after reading your post i thought id try something: I set the option "OSX dark theme at sunset" to enabled, then enabled dark mode in my system preferences, put my computer to sleep and woke it up. As you described, this disabled the dark menubar.

So perhaps you have the same app, or a similar app that also offers an option to manipulate the menubar's state.