This approach of course works. But only as long as one is not using DTTG, too. The global setting for the MD stylesheet is not carried over to DTTG (of course, since one can specify a stylesheet residing outside of DT in the Mac’s filesystem).
Unfortunately, the only way to make a stylesheet work on the desktop and the mobile version of DT is to include it in a DT database and then add a link to it at the top of every MD file like so:
css: x-devonthink-item://0841105D-70B7-4518-8E2C-68E25CF8FC38
where the part after css: is a link to the CSS file in the database. There’s an alternate way with a link element, but this has the same disadvantage: One has to include these links into each and every MD file.
May I therefore humbly suggest that one of the next DTTG versions allows for a global MD style sheet preferences as well (which should preferably be synchronized with DT)? While I’m at it: It would also be nice to be able to specify JavaScript files to be loaded by DT for all MD files. You’re doing that already implicitly with Prism (and possibly Mermaid?) anyway. Making it more flexible would allow for more MD markup without changing the internals of DT (cf. this post: CSS für Devonthink - #13 by chrillek)