Vikunja is a great option, but the mobile app is still in development (lots of missing features). I like it, but I have found that using a simple markdown editor like Markor (on Android) and Marktext (on Linux, available on Windows + Mac too) works for me, and I use Syncthing to sync with my laptop. Note that Marktext isn’t being actively developed anymore, but it was the best one I could find that respects the folder structure of my notes. Obsidian is great too, but it’s closed source and having everything in vaults is annoying for me. You might also like VSCodium or Kate, which are code editors that also have markdown highlighting (but not Markdown preview).

However, you can’t easily use custom themes with Markor and Marktext. For Markor, you can only change the background, font, and text colour of the editing mode (not the preview mode!) while Marktext does let you change the font universally, but you have to use one of the prebuilt themes (I found One Dark to be the closest to my system theme). One day I would like to try my hand at making my own cross-platform markdown editor, maybe once I have less schoolwork to worry about.