I strongly support @Rachik 's suggestions and have been trying to advocate for this for a while, for example in this thread here, where others have also chimed in on this general topic.
I’ve been a professional writer for decades and I love DEVONthink as a tool for my work, except that I absolutely hate writing in it for exactly this reason. It’s incredibly frustrating that all of my documents, notes, and drafts (the latter two all in markdown these days) are housed in a brilliant app but then I never want to use that same app to actually write in. [EDIT: To clarify I am talking here about the markdown editor which is not subject to style-sheet solutions.] I always go through an annoying multi-step process of opening those markdown files from DT into iA Writer and writing there, for exactly the reasons the OP describes. And I then pray that iA Writer will always correctly save my writing back into the file in DT. (So far so good, mostly, though on the iPad I am always holding my breath and can never quite tell if/when iA Writer actually saves back into DTTG.)
So, once again, I will try to register my strong support for what the OP is requesting [EDIT: namely for the markdown editor view]. Yes, it is “only” a seemingly aesthetic matter but for me it has a direct and profound impact on creative focus and process. This is the one thing that still frustrates me to no end about my use of this otherwise incredible app, of which I’ve been an enthusiastic user for many years.