Configurable themes?

No problem to set up an external editor, I didn’t think of using an external editor as a solution to write every note, though I didn’t need Joplin anymore. My point is I try to use Joplin as my journal app. I’m using MWeb now that is good but not freeware, and I’d like to get more freedom from my apps. But MWeb has some smart features, and themes are one of them (not only theme for the rendered markdown, but theme for the interface ot the editor itself, and for the raw markdown). Some apps, free like MacDown and Mou, or not, like MWeb, Byword, iA Writer, Write… have this feature from the beginning. It’s not just an esthetic customisation, it also make the app more comfortable to use to have the good set of colors (as syntax coloration of code editors).
But I follow Joplin fo a while, and I see lot of improvements happen, so maybe this could be a feature in the future roadmap.

Joplin is a note taking app, not an editor. It includes an editor, but its main purpose is to manage notes. However, I agree that the editor could be improved, especially when it comes to customization. But I also think that there are more important things to work on first.

I don't see why not. But as I mentioned before, there are more important things to tackle first.

Note taking app or editor, the question is the time spent on it; and as I choose Joplin first for its taking app capacities, I really hope it'll improve it's design to be more usable (to me, the question of colors of the interface are not only esthetic). Thought I agree with you about priorities, I think the more you use an app, the more comfort become important.
However, as I choose to use markdown rather than wysiwyg interface, I came first here to defend the position to let Joplin be a markdown editor, which is could be uncomfortable for others.
So what is comfortable is not the same for everybody, I fully understand that.

The last usability update was great. A lot of issues have been addressed and fixed. I'm sure usability will be further improved, but not every update can be focused on usability. I definitely see a point in your ideas and I'm confident @laurent will look into and prioritize them.

Good. I’m confident too.