zehyani Hi there!
The AOSP Messaging app that GrapheneOS uses currently doesn't have a dark mode, I'm afraid. The project recently added a rudimentary dark mode to contacts and has added it for other AOSP apps in the past, but changes like that are very low priority.
For larger text, Settings > Accessibility > Display size and text may be what you're looking for.
As for recommendations for 3rd party SMS apps, I don't really have any, I'm afraid. Most well known ones are out of date. If you're okay with using a closed source app, of course, there's always Google Messages.
Just keep in mind that in secondary user profiles, you should use the stock SMS app instead of a different one, as they won't work, if I recall correctly (AOSP issue).
I hope that helps!